More advanced timing loggers.

The new timing loggers have lower overhead since they only push into
a vector. The new format has two types, a start timing and a stop
timing. You can thing of these as brackets associated with a
timestamp. It uses these to construct various statistics when needed,
such as: Total time, exclusive time, and nesting depth.

Changed PrettyDuration to have a default of 3 digits after the decimal
point.

Exaple of a GC dump with exclusive / total times and indenting:
I/art     (23546): GC iteration timing logger [Exclusive time] [Total time]
I/art     (23546):   0ms InitializePhase
I/art     (23546):   0.305ms/167.746ms MarkingPhase
I/art     (23546):     0ms BindBitmaps
I/art     (23546):     0ms FindDefaultSpaceBitmap
I/art     (23546):     0ms/1.709ms ProcessCards
I/art     (23546):       0.183ms ImageModUnionClearCards
I/art     (23546):       0.916ms ZygoteModUnionClearCards
I/art     (23546):       0.610ms AllocSpaceClearCards
I/art     (23546):       1.373ms AllocSpaceClearCards
I/art     (23546):     0.305ms/6.318ms MarkRoots
I/art     (23546):       2.106ms MarkRootsCheckpoint
I/art     (23546):       0.153ms MarkNonThreadRoots
I/art     (23546):       4.287ms MarkConcurrentRoots
I/art     (23546):     43.461ms UpdateAndMarkImageModUnionTable
I/art     (23546):     0ms/112.712ms RecursiveMark
I/art     (23546):       112.712ms ProcessMarkStack
I/art     (23546):     0.610ms/2.777ms PreCleanCards
I/art     (23546):       0.305ms/0.855ms ProcessCards
I/art     (23546):         0.153ms ImageModUnionClearCards
I/art     (23546):         0.610ms ZygoteModUnionClearCards
I/art     (23546):         0.610ms AllocSpaceClearCards
I/art     (23546):         0.549ms AllocSpaceClearCards
I/art     (23546):       0.549ms MarkRootsCheckpoint
I/art     (23546):       0.610ms MarkNonThreadRoots
I/art     (23546):       0ms MarkConcurrentRoots
I/art     (23546):       0.610ms ScanGrayImageSpaceObjects
I/art     (23546):       0.305ms ScanGrayZygoteSpaceObjects
I/art     (23546):       0.305ms ScanGrayAllocSpaceObjects
I/art     (23546):       1.129ms ScanGrayAllocSpaceObjects
I/art     (23546):       0ms ProcessMarkStack
I/art     (23546):   0ms/0.977ms (Paused)PausePhase
I/art     (23546):     0.244ms ReMarkRoots
I/art     (23546):     0.672ms (Paused)ScanGrayObjects
I/art     (23546):     0ms (Paused)ProcessMarkStack
I/art     (23546):     0ms/0.610ms SwapStacks
I/art     (23546):       0.610ms RevokeAllThreadLocalAllocationStacks
I/art     (23546):     0ms PreSweepingGcVerification
I/art     (23546):   0ms/10.621ms ReclaimPhase
I/art     (23546):     0.610ms/0.702ms ProcessReferences
I/art     (23546):       0.214ms/0.641ms EnqueueFinalizerReferences
I/art     (23546):         0.427ms ProcessMarkStack
I/art     (23546):     0.488ms SweepSystemWeaks
I/art     (23546):     0.824ms/9.400ms Sweep
I/art     (23546):       0ms SweepMallocSpace
I/art     (23546):       0.214ms SweepZygoteSpace
I/art     (23546):       0.122ms SweepMallocSpace
I/art     (23546):       6.226ms SweepMallocSpace
I/art     (23546):       0ms SweepMallocSpace
I/art     (23546):       2.144ms SweepLargeObjects
I/art     (23546):     0.305ms SwapBitmaps
I/art     (23546):     0ms UnBindBitmaps
I/art     (23546):   0.275ms FinishPhase
I/art     (23546): GC iteration timing logger: end, 178.971ms

Change-Id: Ia55b65609468f212b3cd65cda66b843da42be645
diff --git a/runtime/utils.h b/runtime/utils.h
index 6d52459..a61d30f 100644
--- a/runtime/utils.h
+++ b/runtime/utils.h
@@ -265,10 +265,11 @@
 // Returns a human-readable time string which prints every nanosecond while trying to limit the
 // number of trailing zeros. Prints using the largest human readable unit up to a second.
 // e.g. "1ms", "1.000000001s", "1.001us"
-std::string PrettyDuration(uint64_t nano_duration);
+std::string PrettyDuration(uint64_t nano_duration, size_t max_fraction_digits = 3);
 
 // Format a nanosecond time to specified units.
-std::string FormatDuration(uint64_t nano_duration, TimeUnit time_unit);
+std::string FormatDuration(uint64_t nano_duration, TimeUnit time_unit,
+                           size_t max_fraction_digits);
 
 // Get the appropriate unit for a nanosecond duration.
 TimeUnit GetAppropriateTimeUnit(uint64_t nano_duration);