Add WebGLContextObject::traceWrappers, and trace its context.

It's not clear to me whether this could be the cause of Issue 671791,
but I can't find any other errors via code inspection.

BUG=671791
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
NOTRY=true

Review-Url: https://codereview.chromium.org/2561733002
Cr-Commit-Position: refs/heads/master@{#437208}

third_party/WebKit/Source/modules/webgl/WebGLContextObject.cpp

@@ -30,7 +30,7 @@
 namespace blink {
 
 WebGLContextObject::WebGLContextObject(WebGLRenderingContextBase* context)
-    : WebGLObject(context), m_context(context) {}
+    : WebGLObject(context), m_context(this, context) {}
 
 bool WebGLContextObject::validate(
     const WebGLContextGroup*,
@@ -55,4 +55,9 @@ DEFINE_TRACE(WebGLContextObject) {
   WebGLObject::trace(visitor);
 }
 
+DEFINE_TRACE_WRAPPERS(WebGLContextObject) {
+  visitor->traceWrappers(m_context);
+  WebGLObject::traceWrappers(visitor);
+}
+
 }  // namespace blink

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

@@ -26,6 +26,7 @@
 #ifndef WebGLContextObject_h
 #define WebGLContextObject_h
 
+#include "bindings/core/v8/TraceWrapperMember.h"
 #include "modules/webgl/WebGLObject.h"
 
 namespace blink {
@@ -43,6 +44,8 @@ class WebGLContextObject : public WebGLObject {
 
   DECLARE_VIRTUAL_TRACE();
 
+  DECLARE_VIRTUAL_TRACE_WRAPPERS();
+
  protected:
   explicit WebGLContextObject(WebGLRenderingContextBase*);
 
@@ -53,7 +56,7 @@ class WebGLContextObject : public WebGLObject {
   gpu::gles2::GLES2Interface* getAGLInterface() const final;
 
  private:
-  Member<WebGLRenderingContextBase> m_context;
+  TraceWrapperMember<WebGLRenderingContextBase> m_context;
 };
 
 }  // namespace blink