Bug 1155025. Make the root layer on fennec have null scroll id with containerless scrolling. r=mstange

On Android we want the root xul document to get a null scroll id so that the root content document gets the first non-null scroll id.

layout/base/nsDisplayList.cpp

@@ -1709,7 +1709,11 @@ already_AddRefed<LayerManager> nsDisplayList::PaintRoot(nsDisplayListBuilder* aB
     } else if (!gfxPrefs::LayoutUseContainersForRootFrames()) {
       // If there is no root scroll frame, and we're using containerless
       // scrolling, pick the document element instead.
+      // On Android we want the root xul document to get a null scroll id
+      // so that the root content document gets the first non-null scroll id.
+#ifndef MOZ_WIDGET_ANDROID
       content = document->GetDocumentElement();
+#endif
     }
 
     root->SetFrameMetrics(