Reset Receiver on WebFrameWidgetBase::Close

IPC messages still might be received while the WebFrameWidgetBase has
been closed but not collected yet. Resetting the receiver prevents
receiving messages in these states.

BUG=1073935

Change-Id: Idef61bae81d142a66efcf5b5ecded7a242383322
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163248Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762003}

third_party/blink/renderer/core/frame/web_frame_widget_base.cc

@@ -107,6 +107,7 @@ void WebFrameWidgetBase::Close(
   request_animation_after_delay_timer_.reset();
   widget_base_->Shutdown(std::move(cleanup_runner), std::move(cleanup_task));
   widget_base_.reset();
+  receiver_.reset();
 }
 
 WebLocalFrame* WebFrameWidgetBase::LocalRoot() const {