DevTools: Cancel async longClick timer when the button gets disabled.

Extracted from https://codereview.chromium.org/23279005/

BUG=
R=caseq@chromium.org, pfeldman@chromium.org, prybin@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23014004

git-svn-id: svn://svn.chromium.org/blink/trunk@156317 bbb929c8-8fbe-4397-9dbb-9b2b20218538