Show the cursor when entering overview.

BUG=309192
TEST=WindowSelectorTest.Basic
TEST=Type some characters and then press the overview button. Mouse cursor
should be visible.

Review URL: https://codereview.chromium.org/37353004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230495 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230495