Disabled form control elements should not be active

Clicking on a disabled form control element like a button should not
make it active. This patch adds a check for the disabled element before
setting the activation. This is as per the spec 
http://www.whatwg.org/specs/web-apps/current-work/multipage/selectors.html#selector-active
This matches the behavior on IE. Firefox behavior is same as Chrome.

BUG=385601
TEST=fast/css/disabled-form-control-elements-should-not-be-active.html

Review URL: https://codereview.chromium.org/340143002

git-svn-id: svn://svn.chromium.org/blink/trunk@176501 bbb929c8-8fbe-4397-9dbb-9b2b20218538