Report DOM partitionAlloc size to V8 GC.

Source/platform/Partitions.cpp

Index: Source/platform/Partitions.cpp
diff --git a/Source/platform/Partitions.cpp b/Source/platform/Partitions.cpp
index 09db484b4cb5d299041b8549e5259985b70910b7..c3c98aba0db8a9740846115883b9edaae49069fa 100644
--- a/Source/platform/Partitions.cpp
+++ b/Source/platform/Partitions.cpp
@@ -35,11 +35,13 @@ namespace WebCore {
 
 SizeSpecificPartitionAllocator<3072> Partitions::m_objectModelAllocator;
 SizeSpecificPartitionAllocator<1024> Partitions::m_renderingAllocator;
+size_t Partitions::m_domMemoryUsage = 0;
 
 void Partitions::init()
 {
     m_objectModelAllocator.init();
     m_renderingAllocator.init();
+    m_domMemoryUsage = 0;
 }
 
 void Partitions::shutdown()