Fix windowless mouse events not working in NPAPI Flash in Chrome desktop Windows AURA.

Flash assumes the container window which is returned via the NPNVnetscapeWindow property corresponds
to the bounds of the webpage. This is not true in AURA where we return the HWND of the main AURA window.
This causes the translations done by NPAPI flash to break.

The fixes are as below:-

1. Create a dummy empty window which corresponds to the bounds of the webpage in the RenderWidgetHostViewAura
   class. We return this window to the plugin when it requests the NPNVnetscapeWindow property.
2. Intercept the WindowFromPoint API and return the dummy parent window to flash.

BUG=301548
R=jam@chromium.org, jam

Review URL: https://codereview.chromium.org/69953030

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235886 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235886