[RemotePlayback] Added IDL harness WebPlatform test for RemotePlayback API.

third_party/WebKit/LayoutTests/external/wpt/interfaces/remoteplayback.idl

+enum RemotePlaybackState {
+    "connecting",
+    "connected",
+    "disconnected"
+};
+
+callback RemotePlaybackAvailabilityCallback = void(boolean available);
+
+interface RemotePlayback : EventTarget {
+    readonly attribute RemotePlaybackState state;
+    attribute EventHandler onconnecting;
+    attribute EventHandler onconnect;
+    attribute EventHandler ondisconnect;
+
+    Promise<long> watchAvailability(RemotePlaybackAvailabilityCallback callback);
+    Promise<void> cancelWatchAvailability(optional long id);
+    Promise<void> prompt();
+};
+
+partial interface HTMLMediaElement {
+    readonly attribute RemotePlayback remote;
+    attribute boolean disableRemotePlayback;
+};