backout Bug 869936 which relies on bug 830347.

widget/windows/winrt/MetroWidget.h

@@ -123,7 +123,6 @@ public:
                                         LayersBackend aBackendHint = mozilla::layers::LAYERS_NONE,
                                         LayerManagerPersistence aPersistence = LAYER_MANAGER_CURRENT,
                                         bool* aAllowRetaining = nullptr);
-  virtual mozilla::layers::LayersBackend GetPreferredCompositorBackend() { return mozilla::layers::LAYERS_D3D11; }
 
   // IME related interfaces
   NS_IMETHOD_(void) SetInputContext(const InputContext& aContext,