Added test to verify [TreatNullAs=EmptyString] behavior of |mediaText|

This patch contains
1. Test added to verify [TreatNullAs=EmptyString] behavior of |mediaText|
   attribute of MediaList.
2. Update MediaList |mediaText| IDL attribute to match the specification.
   https://drafts.csswg.org/cssom/#the-medialist-interface

Bug:

Change-Id: I6cf9380390113e3ec8f37c796b0f6fd68e2de6c8
Reviewed-on: https://chromium-review.googlesource.com/892718
Commit-Queue: Bhagirathi Satpathy <bhagirathi.s@samsung.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532817}

third_party/WebKit/LayoutTests/external/wpt/css/cssom/MediaList.html

@@ -40,6 +40,10 @@
         assert_equals(media[3], undefined, "MediaList indexed getter with out of range after append method");
         assert_equals(media.item(2), "speech", "MediaList item method after append method");
         assert_equals(media.item(3), null, "MediaList item method after append method");
+
+        media.mediaText = null;
+        assert_equals(media.mediaText, "", "MediaList mediaText attribute should be empty string in case of null");
+        assert_equals(media.toString(), "", "MediaList toString method should be empty string in case of null");
     });
     </script>
 </head>

third_party/WebKit/Source/core/css/MediaList.idl

@@ -29,7 +29,7 @@
 [
     Exposed=Window
 ] interface MediaList {
-    [TreatNullAs=EmptyString] stringifier attribute DOMString mediaText;
+    stringifier attribute [TreatNullAs=EmptyString] DOMString mediaText;
     readonly attribute unsigned long length;
     [Measure] getter DOMString? item(unsigned long index);
     // TODO(foolip): appendMedium() and deleteMedium() should never throw.