Adding a class to track when a node in a page gets clicked.

      This is going to be used by the password/form autofill to trigger the
suggestion popup, as part of the effort in moving that code out of WebKit.

BUG=None
TEST=Run unit-tests.

Review URL: http://codereview.chromium.org/3090007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@58597 0039d316-1c4b-4281-b951-d872f2087c98