MacViews: Fix embedded RWHVMac input routing

The method RenderWidgetHostViewMac::GetOffsetFromRootSurface is to be
used to indicate where the view is in its root ui::Compositor. It used
to be that the two were always aligned, but now they can be offset.

Bug: 846018, 	840173
Change-Id: Id3f761d56c75ebd33b75c40c05d46a84b485d162
Reviewed-on: https://chromium-review.googlesource.com/1070883Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561389}

content/browser/renderer_host/render_widget_host_view_mac.mm

@@ -1002,6 +1002,8 @@ bool RenderWidgetHostViewMac::RequestRepaintForTesting() {
 }
 
 gfx::Vector2d RenderWidgetHostViewMac::GetOffsetFromRootSurface() {
+  if (display_only_using_parent_ui_layer_)
+    return view_bounds_in_window_dip_.OffsetFromOrigin();
   return gfx::Vector2d();
 }