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/base/nsGlobalWindow.cpp

@@ -23,6 +23,7 @@
 #include "mozilla/dom/StorageEventBinding.h"
 #include "mozilla/dom/Timeout.h"
 #include "mozilla/dom/TimeoutHandler.h"
+#include "mozilla/dom/VisualViewport.h"
 #include "mozilla/IntegerPrintfMacros.h"
 #include "nsDOMOfflineResourceList.h"
 #include "nsError.h"
@@ -4156,6 +4157,16 @@ nsGlobalWindow::GetNavigator()
   return navigator;
 }
 
+VisualViewport*
+nsGlobalWindow::VisualViewport()
+{
+  if (!mVisualViewport) {
+    mVisualViewport = new mozilla::dom::VisualViewport(this);
+  }
+
+  return mVisualViewport;
+}
+
 nsScreen*
 nsGlobalWindow::GetScreen(ErrorResult& aError)
 {