Issue #2714 - Implement the Visual Viewport API

This DOM/Web API is entirely irrelevant for desktop use.
We give back the width/height of the root scrollframe of content and
for scale we pass forward our dpp resolution (usually 1.0).
Since we have no "no touch" zones in our content on desktop,. the rest
(origin and offset) is hard-coded to (0,0).

dom/webidl/VisualViewport.webidl

@@ -0,0 +1,21 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * The origin of this IDL file is:
+ * https://wicg.github.io/visual-viewport/#the-visualviewport-interface
+ */
+
+interface VisualViewport : EventTarget {
+  readonly attribute double offsetLeft;
+  readonly attribute double offsetTop;
+
+  readonly attribute double pageLeft;
+  readonly attribute double pageTop;
+
+  readonly attribute double width;
+  readonly attribute double height;
+
+  readonly attribute double scale;
+};

dom/webidl/Window.webidl

@@ -15,6 +15,7 @@
  * http://dvcs.w3.org/hg/speech-api/raw-file/tip/speechapi.html
  * https://w3c.github.io/webappsec-secure-contexts/#monkey-patching-global-object
  * https://w3c.github.io/requestidlecallback/
+ * https://wicg.github.io/visual-viewport/#the-visualviewport-interface
  */
 
 interface ApplicationCache;
@@ -492,3 +493,8 @@ partial interface Window {
 dictionary WindowPostMessageOptions : StructuredSerializeOptions {
   USVString targetOrigin = "/";
 };
+
+partial interface Window {
+  [SameObject, Pref="dom.visualviewport.enabled", Replaceable]
+  readonly attribute VisualViewport visualViewport;
+};

dom/webidl/moz.build

@@ -557,6 +557,7 @@ WEBIDL_FILES = [
     'VideoStreamTrack.webidl',
     'VideoTrack.webidl',
     'VideoTrackList.webidl',
+    'VisualViewport.webidl',
     'VTTCue.webidl',
     'VTTRegion.webidl',
     'WaveShaperNode.webidl',