Use strong references from WebGLContextGroup to WebGLRenderingContextBase.

third_party/WebKit/Source/modules/webgl/WebGLContextGroup.h

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 54 matching lines @@
 
   DEFINE_INLINE_TRACE() { visitor->trace(m_contexts); }
 
   DECLARE_VIRTUAL_TRACE_WRAPPERS();
 
  private:
   friend class WebGLObject;
 
   uint32_t m_numberOfContextLosses;
 
-  // Note: ideally this would use a TraceWrapperMember subclass, but
-  // that doesn't seem to exist for weak members yet.
-  HeapHashSet<WeakMember<WebGLRenderingContextBase>> m_contexts;
+  HeapHashSet<TraceWrapperMember<WebGLRenderingContextBase>> m_contexts;
 };
 
 }  // namespace blink
 
 #endif  // WebGLContextGroup_h