[embedlite] Continue using root frame scroll containers for now.

Both android and b2g still use them, only desktop sets the pref to
false. Since with the pref disabled we seem to have multiple problems
when scrolling pages with iframes lets match android/b2g configuration
for now.

embedding/embedlite/embedding.js

@@ -364,3 +364,6 @@ pref("full-screen-api.content-only", true);
 // the window, the window size doesn't change. This pref has no effect when
 // running in actual Metro mode, as the widget will already be fullscreen then.
 pref("full-screen-api.ignore-widgets", true);
+
+// Match defaults for android and b2g, see: modules/libpref/init/all.js
+pref("layout.scroll.root-frame-containers", true);

embedding/embedlite/embedshared/EmbedLiteViewBaseChild.cpp

@@ -710,19 +710,19 @@ EmbedLiteViewBaseChild::RecvUpdateFrame(const FrameMetrics& aFrameMetrics)
   }
 
   if (mViewResized &&
+      aFrameMetrics.GetIsRoot() &&
       mHelper->mLastRootMetrics.GetPresShellId() == aFrameMetrics.GetPresShellId() &&
       mHelper->HandlePossibleViewportChange(mHelper->mInnerSize)) {
     mViewResized = false;
   }
 
   RelayFrameMetrics(aFrameMetrics);
 
-  bool ret = true;
   if (sHandleDefaultAZPC.viewport) {
-    ret = mHelper->RecvUpdateFrame(aFrameMetrics);
+    return mHelper->RecvUpdateFrame(aFrameMetrics);
   }
 
-  return ret;
+  return true;
 }
 
 bool