Support MediaList |toString| method

Added stringifier to support MediaList |toString| method.
Specification:
https://drafts.csswg.org/cssom/#the-medialist-interface

Bug:

Change-Id: I0dd14cd1843a5d3fde424021a443669db374df85
Reviewed-on: https://chromium-review.googlesource.com/890959Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Bhagirathi Satpathy <bhagirathi.s@samsung.com>
Cr-Commit-Position: refs/heads/master@{#532388}

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

@@ -14,6 +14,7 @@
         var media = document.styleSheets[0].cssRules[0].media;
         assert_equals(media.length, 3, "MediaList length attribute");
         assert_equals(media.mediaText, "screen and (min-width: 480px), print, projection", "MediaList mediaText attribute");
+        assert_equals(media.toString(), "screen and (min-width: 480px), print, projection", "MediaList toString method");
         assert_equals(media[0], "screen and (min-width: 480px)", "MediaList indexed getter");
         assert_equals(media[1], "print", "MediaList indexed getter");
         assert_equals(media[2], "projection", "MediaList indexed getter");
@@ -24,6 +25,7 @@
         media.deleteMedium("print");
         assert_equals(media.length, 2, "MediaList length attribute after delete method");
         assert_equals(media.mediaText, "screen and (min-width: 480px), projection", "MediaList mediaText attribute after delete method");
+        assert_equals(media.toString(), "screen and (min-width: 480px), projection", "MediaList toString method after delete method");
         assert_equals(media[1], "projection", "MediaList indexed getter after delete method");
         assert_equals(media[2], undefined, "MediaList indexed getter with out of range after delete method");
         assert_equals(media.item(1), "projection", "MediaList indexed getter after delete method");
@@ -32,6 +34,7 @@
         media.appendMedium("speech");
         assert_equals(media.length, 3, "MediaList length attribute after append method");
         assert_equals(media.mediaText, "screen and (min-width: 480px), projection, speech", "MediaList mediaText attribute after append method");
+        assert_equals(media.toString(), "screen and (min-width: 480px), projection, speech", "MediaList toString method after append method");
         assert_equals(media[1], "projection", "MediaList indexed getter after append method");
         assert_equals(media[2], "speech", "MediaList indexed getter after append method");
         assert_equals(media[3], undefined, "MediaList indexed getter with out of range after append method");

third_party/WebKit/LayoutTests/external/wpt/css/cssom/interfaces-expected.txt

 This is a testharness.js-based test.
-Found 399 tests; 313 PASS, 86 FAIL, 0 TIMEOUT, 0 NOTRUN.
+Found 399 tests; 314 PASS, 85 FAIL, 0 TIMEOUT, 0 NOTRUN.
 PASS Test driver
 PASS HTMLElement interface: attribute style
 PASS HTMLElement interface: style_element must inherit property "style" with the proper type
@@ -27,7 +27,7 @@ PASS MediaList interface object name
 FAIL MediaList interface: existence and properties of interface prototype object assert_equals: prototype of MediaList.prototype is not Array.prototype expected [] but got object "[object Object]"
 PASS MediaList interface: existence and properties of interface prototype object's "constructor" property
 PASS MediaList interface: attribute mediaText
-FAIL MediaList interface: stringifier assert_own_property: interface prototype object missing non-static operation expected property "toString" missing
+PASS MediaList interface: stringifier
 PASS MediaList interface: attribute length
 PASS MediaList interface: operation item(unsigned long)
 PASS MediaList interface: operation appendMedium(CSSOMString)

third_party/WebKit/LayoutTests/fast/dom/domListEnumeration-expected.txt

@@ -74,7 +74,7 @@ PASS resultArray[2].i is '2'
 PASS resultArray[2].item is cssStyleDeclaration.item(2)
 
 [object MediaList]
-PASS resultArray.length is 8
+PASS resultArray.length is 9
 PASS resultArray[0].i is '0'
 PASS resultArray[0].item is mediaList.item(0)
 PASS resultArray[1].i is '1'

third_party/WebKit/LayoutTests/fast/dom/domListEnumeration.html

@@ -191,7 +191,7 @@ shouldBe("resultArray[2].item", "cssStyleDeclaration.item(2)");
 // MediaList
 var mediaList = document.styleSheets[3].cssRules[0].media;
 resultArray = iterateList(mediaList);
-shouldBe("resultArray.length", "8");
+shouldBe("resultArray.length", "9");
 shouldBe("resultArray[0].i", "'0'");
 shouldBe("resultArray[0].item", "mediaList.item(0)");
 shouldBe("resultArray[1].i", "'1'");

third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -3478,6 +3478,7 @@ interface MediaList
     method constructor
     method deleteMedium
     method item
+    method toString
     setter mediaText
 interface MediaQueryList : EventTarget
     attribute @@toStringTag

third_party/WebKit/LayoutTests/webexposed/global-interface-listing-expected.txt

@@ -4156,6 +4156,7 @@ interface MediaList
     method constructor
     method deleteMedium
     method item
+    method toString
     setter mediaText
 interface MediaMetadata
     attribute @@toStringTag

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

@@ -29,8 +29,7 @@
 [
     Exposed=Window
 ] interface MediaList {
-    // TODO(foolip): [TreatNullAs=EmptyString] stringifier attribute DOMString mediaText;
-    attribute DOMString? mediaText;
+    [TreatNullAs=EmptyString] stringifier attribute DOMString mediaText;
     readonly attribute unsigned long length;
     [Measure] getter DOMString? item(unsigned long index);
     // TODO(foolip): appendMedium() and deleteMedium() should never throw.