Experiment: assert for stack-allocated GarbageCollected<>s.

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

Index: third_party/WebKit/Source/platform/heap/Heap.h
diff --git a/third_party/WebKit/Source/platform/heap/Heap.h b/third_party/WebKit/Source/platform/heap/Heap.h
index 24dd654eb92b1ae47477925a87d3867f6f72798c..87d01fcc8e5a79bce2283acb02b3b0c93689405d 100644
--- a/third_party/WebKit/Source/platform/heap/Heap.h
+++ b/third_party/WebKit/Source/platform/heap/Heap.h
@@ -35,6 +35,7 @@
 #include "platform/heap/GCInfo.h"
 #include "platform/heap/HeapPage.h"
 #include "platform/heap/PageMemory.h"
+#include "platform/heap/StackFrameDepth.h"
 #include "platform/heap/ThreadState.h"
 #include "platform/heap/Visitor.h"
 #include "wtf/AddressSanitizer.h"
@@ -474,6 +475,11 @@ public:
 protected:
     GarbageCollected()
     {
+        // A GarbageCollected<> object should not be stack allocated
+        // as it won't be prefixed with a (heap) header which would
+        // fail if the stack object ends up being stored in a traceable
+        // member field (say.)
+        DCHECK(!StackFrameDepth::isStackAddress(this));
     }
 };