Merge the Merlin heap tracing to top-of-trunk.

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
diff --git a/runtime/vm/pages.cc b/runtime/vm/pages.cc
index 0aca3477a69152c206791224f90ddbe05b1ba7c3..a37bc24d9390f8b782f10917de9d9bf271e0c922 100644
--- a/runtime/vm/pages.cc
+++ b/runtime/vm/pages.cc
@@ -8,6 +8,7 @@
 #include "vm/compiler_stats.h"
 #include "vm/gc_marker.h"
 #include "vm/gc_sweeper.h"
+#include "vm/heap_trace.h"
 #include "vm/object.h"
 #include "vm/virtual_memory.h"
 
@@ -406,6 +407,10 @@ void PageSpace::MarkSweep(bool invoke_api_callbacks, const char* gc_reason) {
   Isolate* isolate = Isolate::Current();
   NoHandleScope no_handles(isolate);
 
+  if (HeapTrace::is_enabled()) {
+    isolate->heap()->trace()->TraceMarkSweepStart();
+  }
+
   if (FLAG_print_free_list_before_gc) {
     OS::Print("Data Freelist (before GC):\n");
     freelist_[HeapPage::kData].Print();
@@ -501,6 +506,10 @@ void PageSpace::MarkSweep(bool invoke_api_callbacks, const char* gc_reason) {
     OS::PrintErr(" done.\n");
   }
 
+  if (HeapTrace::is_enabled()) {
+    isolate->heap()->trace()->TraceMarkSweepFinish();
+  }
+
   count_++;
   // Done, reset the marker.
   ASSERT(sweeping_);