Clean up sampling tracing.

- Moved maps to fields within thread.
- Created temp trace field to lessen amount of traces allocated and freed.

Change-Id: I23fe25a85ad2894cc6917f87d4046cdececf0739
diff --git a/runtime/thread.cc b/runtime/thread.cc
index c4077de..8fc744d 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -961,6 +961,8 @@
       jpeer_(NULL),
       stack_begin_(NULL),
       stack_size_(0),
+      stack_trace_sample_(NULL),
+      trace_clock_base_(0),
       thin_lock_id_(0),
       tid_(0),
       wait_mutex_(new Mutex("a thread wait mutex")),
@@ -1077,6 +1079,7 @@
   delete debug_invoke_req_;
   delete instrumentation_stack_;
   delete name_;
+  delete stack_trace_sample_;
 
   TearDownAlternateSignalStack();
 }