Add a flag for EmbedderGraph in heap snapshot.

The flag is enabled by default and provides a quick way to switch
between the old RetainerInfo and the new EmbedderGraph snapshoting
in local testing.

Bug: chromium:749490
Change-Id: I36406597a289090879cfa5051037c8cf35988e59
Reviewed-on: https://chromium-review.googlesource.com/897532Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51045}

src/flag-definitions.h

@@ -910,7 +910,8 @@ DEFINE_INT(histogram_interval, 600000,
 // heap-snapshot-generator.cc
 DEFINE_BOOL(heap_profiler_trace_objects, false,
             "Dump heap object allocations/movements/size_updates")
-
+DEFINE_BOOL(heap_profiler_use_embedder_graph, true,
+            "Use the new EmbedderGraph API to get embedder nodes")
 
 // sampling-heap-profiler.cc
 DEFINE_BOOL(sampling_heap_profiler_suppress_randomness, false,

src/profiler/heap-snapshot-generator.cc

@@ -2223,7 +2223,8 @@ bool NativeObjectsExplorer::IterateAndExtractReferences(
     SnapshotFiller* filler) {
   filler_ = filler;
 
-  if (snapshot_->profiler()->HasBuildEmbedderGraphCallback()) {
+  if (FLAG_heap_profiler_use_embedder_graph &&
+      snapshot_->profiler()->HasBuildEmbedderGraphCallback()) {
     v8::HandleScope scope(reinterpret_cast<v8::Isolate*>(isolate_));
     DisallowHeapAllocation no_allocation;
     EmbedderGraphImpl graph;

test/cctest/test-heap-profiler.cc

@@ -2878,6 +2878,7 @@ void CheckEmbedderGraphSnapshot(v8::Isolate* isolate,
 }
 
 TEST(EmbedderGraph) {
+  i::FLAG_heap_profiler_use_embedder_graph = true;
   LocalContext env;
   v8::HandleScope scope(env->GetIsolate());
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(env->GetIsolate());