Notifications: icon url is USVString.

See: https://notifications.spec.whatwg.org/#api

Review URL: https://codereview.chromium.org/1633123002

Cr-Commit-Position: refs/heads/master@{#371514}

third_party/WebKit/Source/modules/notifications/Notification.idl

@@ -65,7 +65,7 @@ enum NotificationPermission {
     readonly attribute DOMString lang;
     readonly attribute DOMString body;
     readonly attribute DOMString tag;
-    readonly attribute DOMString icon;
+    readonly attribute USVString icon;
 
     [RuntimeEnabled=NotificationExperimental] readonly attribute sequence<unsigned long>? vibrate;
     readonly attribute boolean silent;

third_party/WebKit/Source/modules/notifications/NotificationOptions.idl

@@ -15,7 +15,7 @@ dictionary NotificationOptions {
     DOMString lang = "";
     DOMString body = "";
     DOMString tag = "";
-    DOMString icon;
+    USVString icon;
     // TODO(sh919.park): vibrate should be ([Clamp] unsigned long or sequence<unsigned long>)
     (unsigned long or sequence<unsigned long>) vibrate;
     boolean silent = false;