[tracing] Adding class-wise memory statistics to blink gc memory dumps

Source/platform/heap/ThreadState.h

Index: Source/platform/heap/ThreadState.h
diff --git a/Source/platform/heap/ThreadState.h b/Source/platform/heap/ThreadState.h
index 56f9e15ee73462e93a0af440a557b1c91f5cd442..61dcf31e7e6683b7985b3abefdc51e011cbd9a47 100644
--- a/Source/platform/heap/ThreadState.h
+++ b/Source/platform/heap/ThreadState.h
@@ -489,6 +489,20 @@ public:
     // Visit all persistents allocated on this thread.
     void visitPersistents(Visitor*);
 
+    struct GCSnapshotInfo {
+        // Map from base-classes to a snapshot class-ids (used as index below).
+        using ClassTagMap = HashMap<const GCInfo*, size_t>;

[haraken, 2015/06/26 05:35:28]

How about just using gcInfoIndex for the tag? gcInfoIndex is already a tag for
GCInfo.

[ssid, 2015/06/26 10:10:28]

Hm, I think it is better to store map of GcInfo* here since everytime
takeSnapshot() method is called it needs to get GcInfo from GcInfoIndex. 
ie, instead of key->m_className i would have to do Heap::gcInfo(key)->className.

This adds table look ups everytime dump is taken.

+        ClassTagMap classTags;
+
+        // Map from class-id (index) to count/size.
+        Vector<int> liveCount;
+        Vector<int> deadCount;
+        Vector<size_t> liveSize;
+        Vector<size_t> deadSize;
+
+        size_t getClassTag(const GCInfo*);
+    };
+
 #if ENABLE(GC_PROFILING)
     const GCInfo* findGCInfo(Address);
     static const GCInfo* findGCInfoFromAllThreads(Address);