Rename RootWindow -> WindowEventDispatcher.

http://crbug.com/308843

R=sky@chromium.org

Review URL: https://codereview.chromium.org/172743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252501 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252501