Media Remoting: Add interstitial elements to media element shadow dom.

third_party/WebKit/Source/core/css/CSSValueKeywords.json5

 {
   // The mode argument is used to limit the keyword to be used only for certain
   // CSSParserModes. Values that have the prefix -internal- are only allowed by
   // CSSParserModes listed in allowInternalPropertyAndValue()
 
   parameters: {
     mode: {},
   },
 
   //
@@ skipping 650 matching lines @@
     "media-controls-fullscreen-background",
     "media-current-time-display",
     "media-time-remaining-display",
     "-internal-media-cast-off-button",
     "-internal-media-overlay-cast-off-button",
     "-internal-media-track-selection-checkmark",
     "-internal-media-closed-captions-icon",
     "-internal-media-subtitles-icon",
     "-internal-media-overflow-button",
     "-internal-media-download-button",
+    "-internal-media-remoting-cast-icon",
     "menulist",
     "menulist-button",
     "menulist-text",
     "menulist-textfield",
     "meter",
     "progress-bar",
     "progress-bar-value",
     "slider-horizontal",
     "slider-vertical",
     "sliderthumb-horizontal",
@@ skipping 452 matching lines @@
     // shape
     // rect
     // round
 
     // color-gamut
     // srgb
     "p3",
     "rec2020",
   ],
 }