Make MediaSession per frame as an attribute of Navigator

third_party/WebKit/LayoutTests/media/mediasession/mediaimage.html

Index: third_party/WebKit/LayoutTests/media/mediasession/mediaimage.html
diff --git a/third_party/WebKit/LayoutTests/media/mediasession/mediaimage.html b/third_party/WebKit/LayoutTests/media/mediasession/mediaimage.html
index dcf76e5f5db2783c7db52642014cee1a921341e2..4effd81d8db24e8111c5c6ca1e3b383f19a1f9f7 100644
--- a/third_party/WebKit/LayoutTests/media/mediasession/mediaimage.html
+++ b/third_party/WebKit/LayoutTests/media/mediasession/mediaimage.html
@@ -3,11 +3,9 @@
 <script src="../../resources/testharness.js"></script>
 <script src="../../resources/testharnessreport.js"></script>
 <script>
-  console.warn("dummy log entry to avoid an empty -expected.txt in virtual/");
-
   test(function() {
     var image = new MediaImage({});
-    assert_true(image != null);
+    assert_not_equals(image, null);
 
     var exception = false;
     try {