Use strong references from WebGLContextGroup to WebGLRenderingContextBase.

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

Index: third_party/WebKit/Source/modules/webgl/WebGLContextGroup.h
diff --git a/third_party/WebKit/Source/modules/webgl/WebGLContextGroup.h b/third_party/WebKit/Source/modules/webgl/WebGLContextGroup.h
index 4986a45d53dba7707731baebcbc19cda1a43ea10..8884a063ac7abdcf287a7572b065dad312d88fec 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGLContextGroup.h
+++ b/third_party/WebKit/Source/modules/webgl/WebGLContextGroup.h
@@ -72,9 +72,7 @@ class WebGLContextGroup final : public GarbageCollected<WebGLContextGroup>,
 
   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