Bug 1060918 - Do not rely on two implicit conversions in GLContextProviderWGL.cpp for clang-cl compatibility; r=jrmuizel

This works around http://llvm.org/PR20821.

gfx/gl/GLContextProviderWGL.cpp

@@ -635,7 +635,7 @@ GLContextProviderWGL::CreateHeadless()
         return nullptr;
     }
 
-    nsRefPtr<GLContext> retGL = glContext;
+    nsRefPtr<GLContext> retGL = glContext.get();
     return retGL.forget();
 }