Avoid PageMemoryRegion::m_numPages data race.

third_party/WebKit/Source/platform/heap/PageMemory.cpp

Index: third_party/WebKit/Source/platform/heap/PageMemory.cpp
diff --git a/third_party/WebKit/Source/platform/heap/PageMemory.cpp b/third_party/WebKit/Source/platform/heap/PageMemory.cpp
index 5efe1fcede7eb3036afbf20179a5dcfb76e7911d..d5d4cdf26b0e238919640d10a2a5e7ef53fec043 100644
--- a/third_party/WebKit/Source/platform/heap/PageMemory.cpp
+++ b/third_party/WebKit/Source/platform/heap/PageMemory.cpp
@@ -6,6 +6,7 @@
 
 #include "platform/heap/Heap.h"
 #include "wtf/Assertions.h"
+#include "wtf/Atomics.h"
 #include "wtf/PageAllocator.h"
 
 namespace blink {
@@ -44,6 +45,13 @@ PageMemoryRegion::~PageMemoryRegion()
     release();
 }
 
+void PageMemoryRegion::pageDeleted(Address page)
+{
+    markPageUnused(page);
+    if (!atomicDecrement(&m_numPages))
+        delete this;
+}
+
 // TODO(haraken): Like partitionOutOfMemoryWithLotsOfUncommitedPages(),
 // we should probably have a way to distinguish physical memory OOM from
 // virtual address space OOM.