Provide access to the WindowEventDispatcher as a ui::EventProcessor.

This avoids using WED from a number of places and moves us closer to a world where WED is more of an implementation detail of WTH.

R=sky@chromium.org
TBR=sky@chromium.org
http://crbug.com/308843

Review URL: https://codereview.chromium.org/191153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256053 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256053