[ MimeHandlerView ] Minor cleanups

Removing unused methods and includes.

TBR=wjmaclean@chromium.org

Bug: None
Change-Id: I2464652110bfa2fd9a2749d48c9ddcca5add92b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1543449Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645308}

extensions/renderer/guest_view/mime_handler_view/mime_handler_view_frame_container.cc

@@ -6,8 +6,6 @@
 
 #include <string>
 
-#include "base/containers/flat_set.h"
-#include "base/pickle.h"
 #include "content/public/common/webplugininfo.h"
 #include "content/public/renderer/render_frame.h"
 #include "content/public/renderer/render_frame_observer.h"

extensions/renderer/guest_view/mime_handler_view/mime_handler_view_frame_container.h

@@ -79,8 +79,6 @@ class MimeHandlerViewFrameContainer : public MimeHandlerViewContainerBase {
 
   blink::WebFrame* GetContentFrame() const;
 
-  void OnMessageReceived(const IPC::Message& message);
-
   blink::WebElement plugin_element_;
   const int32_t element_instance_id_;
   std::unique_ptr<RenderFrameLifetimeObserver> render_frame_lifetime_observer_;