Report DOM partitionAlloc size to V8 GC.

Source/platform/Partitions.h

Index: Source/platform/Partitions.h
diff --git a/Source/platform/Partitions.h b/Source/platform/Partitions.h
index c589220f5fca4620c9c167a17dc47939030f9bab..e1dd2eddd4ce95c6e2367eabd2c8bce62d4d388e 100644
--- a/Source/platform/Partitions.h
+++ b/Source/platform/Partitions.h
@@ -44,6 +44,11 @@ public:
     ALWAYS_INLINE static PartitionRoot* getObjectModelPartition() { return m_objectModelAllocator.root(); }
     ALWAYS_INLINE static PartitionRoot* getRenderingPartition() { return m_renderingAllocator.root(); }
 
+    static size_t currentDOMMemoryUsage()
+    {
+        return m_objectModelAllocator.root()->totalSizeOfCommittedPages;
+    }
+
 private:
     static SizeSpecificPartitionAllocator<3072> m_objectModelAllocator;
     static SizeSpecificPartitionAllocator<1024> m_renderingAllocator;