Add input observer to RenderWidgetHosts from Frame notifications

This covers the case of a RenderWidgetHost for a subframe, for which we
do not receive View notifications.

Bug: b/148303679
Change-Id: Iaaf6748b0a40e37e83d8b54925666f254a8b392d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037635Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Kevin Schoedel <kpschoedel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738319}

chromecast/browser/webview/web_content_controller.cc

@@ -465,11 +465,14 @@ void WebContentController::RenderFrameCreated(
   // it later on.
   if (instance)
     SendInitialChannelSet(instance);
+  ObserveRenderWidget(render_frame_host->GetView()->GetRenderWidgetHost());
 }
 
 void WebContentController::RenderFrameDeleted(
     content::RenderFrameHost* render_frame_host) {
   current_render_frame_set_.erase(render_frame_host);
+  current_render_widget_set_.erase(
+      render_frame_host->GetView()->GetRenderWidgetHost());
 }
 
 void WebContentController::RenderFrameHostChanged(