Verify that a new heap page isn't also marked as being off heap.

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

Index: third_party/WebKit/Source/platform/heap/PageMemory.h
diff --git a/third_party/WebKit/Source/platform/heap/PageMemory.h b/third_party/WebKit/Source/platform/heap/PageMemory.h
index 6e1e87a24d91de89b067376e31b5d8f808f8808b..2407eed99491efd485ad97e10ab89ddc9a348d48 100644
--- a/third_party/WebKit/Source/platform/heap/PageMemory.h
+++ b/third_party/WebKit/Source/platform/heap/PageMemory.h
@@ -174,6 +174,15 @@ class PageMemory {
 
   WARN_UNUSED_RESULT bool commit() {
     m_reserved->markPageUsed(writableStart());
+    // Check that in-use page isn't also marked as being a non-heap page
+    // by the current heap's negative cache. That cache is invalidated
+    // when allocating new pages, but crbug.com/649485 suggests that
+    // we do get out of sync somehow.
+    //
+    // TODO(sof): consider removing check once bug has been diagnosed
+    // and addressed.
+    CHECK(!ThreadState::current()->isAddressInHeapDoesNotContainCache(
+        writableStart()));
     return m_writable.commit();
   }