Avoid unsafe heap access from audio thread.

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

Index: third_party/WebKit/Source/platform/heap/ThreadState.h
diff --git a/third_party/WebKit/Source/platform/heap/ThreadState.h b/third_party/WebKit/Source/platform/heap/ThreadState.h
index dd5f2218dfcf8884e7577a843c209b61c1b96b63..fd6b792d1213261630714c01399b97fff51b042e 100644
--- a/third_party/WebKit/Source/platform/heap/ThreadState.h
+++ b/third_party/WebKit/Source/platform/heap/ThreadState.h
@@ -320,6 +320,23 @@ class PLATFORM_EXPORT ThreadState {
     ThreadState* const thread_state_;
   };
 
+  // Per-thread lock that's held while a GC runs; can be
+  // used by non-attached threads to safely coordinate access
+  // to the heap while it is in a non-GCing state.

[hongchan, 2017/06/01 16:08:55]

From what I understand, this should not be abused and the general policy should
be applied for most cases. (i.e. don't touch other threads' objects.)

Perhaps we need some warnings here?

+  class GCLockScope final {
+    STACK_ALLOCATED();
+
+   public:
+    explicit GCLockScope(ThreadState* thread_state)
+        : thread_state_(thread_state) {
+      thread_state_->active_gc_mutex_.lock();
+    }
+    ~GCLockScope() { thread_state_->active_gc_mutex_.unlock(); }
+
+   private:
+    ThreadState* const thread_state_;
+  };
+
   void FlushHeapDoesNotContainCacheIfNeeded();
 
   // Safepoint related functionality.
@@ -629,6 +646,7 @@ class PLATFORM_EXPORT ThreadState {
   void ReportMemoryToV8();
 
   friend class SafePointScope;
+  friend class GCLockScope;
 
   static WTF::ThreadSpecific<ThreadState*>* thread_specific_;
 
@@ -656,6 +674,7 @@ class PLATFORM_EXPORT ThreadState {
   size_t mixins_being_constructed_count_;
   double accumulated_sweeping_time_;
   bool object_resurrection_forbidden_;
+  Mutex active_gc_mutex_;
 
   BaseArena* arenas_[BlinkGC::kNumberOfArenas];
   int vector_backing_arena_index_;