Add autoclick accessibility feature to ash.

BUG=272401
TEST=added unit tests

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=226895

Review URL: https://codereview.chromium.org/24191002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227760 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227760