[Media Session] Convert to weak ptr

Convert the GetMediaImageBitmap to use a weak ptr of
the ImageObserverHolder to fix a crash.

BUG=982073

Change-Id: I3f97e778d2c99e55bdc4287c8073a6f3639d069a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692219Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675772}

services/media_session/media_controller.cc

@@ -6,6 +6,7 @@
 
 #include <set>
 
+#include "base/memory/weak_ptr.h"
 #include "services/media_session/audio_focus_request.h"
 #include "services/media_session/public/cpp/media_image_manager.h"
 
@@ -56,7 +57,7 @@ class MediaController::ImageObserverHolder {
     owner_->session_->ipc()->GetMediaImageBitmap(
         *image, minimum_size_px_, desired_size_px_,
         base::BindOnce(&MediaController::ImageObserverHolder::OnImage,
-                       base::Unretained(this)));
+                       weak_ptr_factory_.GetWeakPtr()));
   }
 
   void ClearImage() {
@@ -80,6 +81,8 @@ class MediaController::ImageObserverHolder {
 
   mojom::MediaControllerImageObserverPtr observer_;
 
+  base::WeakPtrFactory<ImageObserverHolder> weak_ptr_factory_{this};
+
   DISALLOW_COPY_AND_ASSIGN(ImageObserverHolder);
 };