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..bd5ef67a8ee5c239a3a638d7b8aa848ecfe398cc 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 +6925,37 @@ bool Isolate::GetHeapSpaceStatistics(HeapSpaceStatistics* space_statistics,
 }
 
 
+size_t Isolate::NumberOfHeapObjectsTypes() {

[ssid, 2015/05/05 14:26:16]

primiano@ here is the definition.

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

[Primiano Tucci (use gerrit), 2015/05/05 14:17:53]

Is object type ever expected to be null? If so why does it imply that object
count should be 0? Maybe add a comment

[ssid, 2015/05/05 14:26:16]

The object type can be null in some cases, there are 241 object types defined in
multiple macros. but they are not continuous sequences.
DCHECK here is to check if no object type exists that is counted but not named.
will write a comment.

+    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);