[Files.app] Add a test of the focus behavior around the search box

This test checks the following:
- File list is focused on launch
- On pressing Ctrl-F, the focus goes to the search box.
- On pressing Tab, the focus goes back to the file list

BUG=305506
TEST=browser_tests pass
R=hirono@chromium.org, mtomasz@chromium.org

Review URL: https://codereview.chromium.org/26674002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227860 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227860