Media Session: use dictionary instead of an interface for MediaImage.

third_party/WebKit/Source/modules/mediasession/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/WebKit/Source/modules/modules.gni")
 
 blink_modules_sources("mediasession") {
   sources = [
-    "MediaImage.cpp",
-    "MediaImage.h",
     "MediaMetadata.cpp",
     "MediaMetadata.h",
     "MediaMetadataSanitizer.cpp",
     "MediaMetadataSanitizer.h",
     "MediaSession.cpp",
     "MediaSession.h",
     "NavigatorMediaSession.cpp",
     "NavigatorMediaSession.h",
   ]
 }