Bug 795811 - Make GetPrimaryScrollableLayer() return null on all configurations except MOZ_WIDGET_ANDROID r=cjones

gfx/layers/Layers.cpp

@@ -200,6 +200,12 @@ namespace layers {
 Layer*
 LayerManager::GetPrimaryScrollableLayer()
 {
+  // AsyncPanZoomController handles multiple scrollable layers, so there is no
+  // "primary" scrollable layer.
+#ifndef MOZ_WIDGET_ANDROID
+  return nullptr;
+#endif
+
   if (!mRoot) {
     return nullptr;
   }