focus reverse traversal was not working for TextfieldViews.

 Both Textfield and TextfieldViews were focusable and
alt-tab was setting the focus back to the Textfield, which
sets the focus to the its TextfieldViews. This fixes the issue
by making Textfield non focusable.

BUG=none
TEST=FocusTest is added to unittest.

Review URL: http://codereview.chromium.org/5988010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@70624 0039d316-1c4b-4281-b951-d872f2087c98