Check isUserActionElement() instead of separate hover/active/focus checks in supportsStyleSharing()

Since we check all the different states of being a user action element we
should just check the primary bit instead. This function is super hot so
avoiding some branches helps.

This will make us not share for elements in the "active chain", but in the 
common case that's the same as the regular :active elements unless you mutate
the elements in the :active state inside an action (ex. click)

Review URL: https://codereview.chromium.org/236153005

git-svn-id: svn://svn.chromium.org/blink/trunk@171435 bbb929c8-8fbe-4397-9dbb-9b2b20218538