Avoid unsafe heap access from audio thread.

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

Index: third_party/WebKit/Source/platform/heap/ThreadState.cpp
diff --git a/third_party/WebKit/Source/platform/heap/ThreadState.cpp b/third_party/WebKit/Source/platform/heap/ThreadState.cpp
index 58dc75b9c006da15a9ecd07cde9ade0483e4069e..e1aa938312f7f4cd1a2581bc19b384941ed123c7 100644
--- a/third_party/WebKit/Source/platform/heap/ThreadState.cpp
+++ b/third_party/WebKit/Source/platform/heap/ThreadState.cpp
@@ -1451,6 +1451,12 @@ void ThreadState::CollectGarbage(BlinkGC::StackState stack_state,
   GCForbiddenScope gc_forbidden_scope(this);
 
   {
+    // No access to GCing arenas is allowed by other threads while
+    // a GC runs; grab a lock to indicate that GC is underway. Should
+    // any other thread need to access object on the heap, it will have
+    // to be while in a GCLockScope also.
+    GCLockScope gc_lock(this);
+
     // Access to the CrossThreadPersistentRegion has to be prevented while in
     // the marking phase because otherwise other threads may allocate or free
     // PersistentNodes and we can't handle that.