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 e6c4b378145fa7f96fe5a9423f9d279ab6a834e6..a0d75fcc36c610b0fb1a728a9d0526c65b72a25c 100644
--- a/runtime/vm/pages.cc
+++ b/runtime/vm/pages.cc
@@ -7,6 +7,7 @@
 #include "platform/assert.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"
 
@@ -399,6 +400,10 @@ 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:\n");
     freelist_[HeapPage::kData].Print();
@@ -494,6 +499,10 @@ 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_);