Add autoclick to settings page under the accessibility section.

In the settings page, we have both a checkbox to enable/disable the feature and a dropdown menu to choose the delay. This feature is hidden behind a flag.

BUG=272401
TEST=manual

Review URL: https://codereview.chromium.org/27196006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230075 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230075