Adding api to get last gc object statistics for chrome://tracing.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 9986df8bf2dd399a8ed836af7b6b353a5221d59c..ddd10309a1b2ea249380590d8f1f83080ae3c4e2 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -5348,6 +5348,10 @@ HeapSpaceStatistics::HeapSpaceStatistics(): space_name_(0),
                                             physical_space_size_(0) { }
 
 
+HeapObjectStatistics::HeapObjectStatistics(): object_type_(nullptr),
+                                              object_count_(0),
+                                              object_size_(0) { }
+
 bool v8::V8::InitializeICU(const char* icu_data_file) {
   return i::InitializeICU(icu_data_file);
 }
@@ -6921,6 +6925,38 @@ bool Isolate::GetHeapSpaceStatistics(HeapSpaceStatistics* space_statistics,
 }
 
 
+size_t Isolate::NumberOfHeapObjectsTypes() {
+  return i::Heap::OBJECT_STATS_COUNT;
+}

[ulan, 2015/05/06 12:46:02]

Two new-lines between functions. Please run "git cl format" to fix style.

[ssid, 2015/05/07 13:53:03]

Done.

+
+bool Isolate::GetLastGcObjectStatistics(HeapObjectStatistics* object_statistics,
+                                        size_t type_index) {
+  if (!object_statistics)
+    return false;

[ulan, 2015/05/06 12:46:02]

Either use {} or put return statements in the previous line.

[ssid, 2015/05/07 13:53:03]

Done.

+  if (type_index >= i::Heap::OBJECT_STATS_COUNT)
+    return false;
+  if (!i::FLAG_track_gc_object_stats)
+    return false;
+
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  i::Heap* heap = isolate->heap();
+  const char* object_type = heap->GetObjectTypeName(type_index);
+  size_t object_count = heap->object_count_last_gc(type_index);
+  size_t object_size = heap->object_size_last_gc(type_index);
+  if (object_type == nullptr) {

[ulan, 2015/05/06 12:46:02]

This case shouldn't be happening. Could you please post here which index has no
type?

[ssid, 2015/05/07 13:53:03]

As discussed offline, there are string categories that are not tracked/named.
So, this DCHECK helps to check if the objects are actually tracked.

+    // There should be no objects counted when the type is unknown.
+    DCHECK_EQ(object_count, 0);
+    DCHECK_EQ(object_size, 0);
+    return false;
+  }
+
+  object_statistics->object_type_ = object_type;
+  object_statistics->object_count_ = object_count;
+  object_statistics->object_size_ = object_size;
+  return true;
+}
+
+
 void Isolate::GetStackSample(const RegisterState& state, void** frames,
                              size_t frames_limit, SampleInfo* sample_info) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);