Fix a methods dec. in MimeHandlerViewContainerBase

The method CreateMimeHandlerViewGuestIfNecessary() has an implementation
in mime_handler_view_container_base.cc. It is marked as abstract by
mistake.

TBR=wjmaclean@chromium.org

Bug: 659750
Change-Id: If4751752f05ff120f47c7d65a5a6c3189aa08ac3
Reviewed-on: https://chromium-review.googlesource.com/1184119Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585007}

extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container_base.h

@@ -60,7 +60,7 @@ class MimeHandlerViewContainerBase : public blink::WebAssociatedURLLoaderClient,
  protected:
   // Returns the frame which is embedding the corresponding plugin element.
   virtual content::RenderFrame* GetEmbedderRenderFrame() const;
-  virtual void CreateMimeHandlerViewGuestIfNecessary() = 0;
+  virtual void CreateMimeHandlerViewGuestIfNecessary();
   virtual blink::WebFrame* GetGuestProxyFrame() const = 0;
   virtual int32_t GetInstanceId() const = 0;
   virtual gfx::Size GetElementSize() const = 0;