SkThreadPool ~~> SkTaskGroup

SkTaskGroup is like SkThreadPool except the threads stay in
one global pool.  Each SkTaskGroup itself is tiny (4 bytes)
and its wait() method applies only to tasks add()ed to that
instance, not the whole thread pool.

This means we don't need to bring up new thread pools when
tests themselves want to use multithreading (e.g. pathops,
quilt).  We just create a new SkTaskGroup and wait for that
to complete.  This should be more efficient, and allow us
to expand where we use threads to really latency sensitive
places.  E.g. we can probably now use these in nanobench
for CPU .skp rendering.

Now that all threads are sharing the same pool, I think we
can remove most of the custom mechanism pathops tests use
to control threading.  They'll just ride on the global pool
with all other tests now.

This (temporarily?) removes the GPU multithreading feature
from DM, which we don't use.

On my desktop, DM runs a little faster (57s -> 55s) in
Debug, and a lot faster in Release (36s -> 24s).  The bots
show speedups of similar proportions, cutting more than a
minute off the N4/Release and Win7/Debug runtimes.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/9c7207b5dc71dc5a96a2eb107d401133333d5b6f

R=caryclark@google.com, bsalomon@google.com, bungeman@google.com, mtklein@google.com, reed@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/531653002
diff --git a/tests/skia_test.cpp b/tests/skia_test.cpp
index 97ac29b..0058215 100644
--- a/tests/skia_test.cpp
+++ b/tests/skia_test.cpp
@@ -12,8 +12,8 @@
 #include "SkGraphics.h"
 #include "SkOSFile.h"
 #include "SkTArray.h"
+#include "SkTaskGroup.h"
 #include "SkTemplates.h"
-#include "SkThreadPool.h"
 #include "SkTime.h"
 #include "Test.h"
 
@@ -54,7 +54,6 @@
     explicit DebugfReporter(int total) : fDone(0), fTotal(total) {}
 
     virtual bool allowExtendedTest() const SK_OVERRIDE { return FLAGS_extendedTest; }
-    virtual bool allowThreaded()     const SK_OVERRIDE { return !FLAGS_single; }
     virtual bool verbose()           const SK_OVERRIDE { return FLAGS_veryVerbose; }
 
 protected:
@@ -175,7 +174,8 @@
     int32_t failCount = 0;
     int skipCount = 0;
 
-    SkThreadPool threadpool(FLAGS_threads);
+    SkTaskGroup::Enabler enabled(FLAGS_threads);
+    SkTaskGroup cpuTests;
     SkTArray<Test*> gpuTests;  // Always passes ownership to an SkTestRunnable
 
     DebugfReporter reporter(toRun);
@@ -186,7 +186,7 @@
         } else if (test->isGPUTest()) {
             gpuTests.push_back() = test.detach();
         } else {
-            threadpool.add(SkNEW_ARGS(SkTestRunnable, (test.detach(), &failCount)));
+            cpuTests.add(SkNEW_ARGS(SkTestRunnable, (test.detach(), &failCount)));
         }
     }
 
@@ -204,7 +204,7 @@
     }
 
     // Block until threaded tests finish.
-    threadpool.wait();
+    cpuTests.wait();
 
     if (FLAGS_verbose) {
         SkDebugf("\nFinished %d tests, %d failures, %d skipped. (%d internal tests)",