Be more strict about creating user gestures for plugins.

We should only create a user gesture if the current input event matches
the handled event. Also, we should not create a new user gesture, in
case there is already a consumed user gesture on the stack.

BUG=350831
TEST=plugins/user-gesture.html, fast/events/popup-blocked-from-wrong-event.html

Review URL: https://codereview.chromium.org/192593002

git-svn-id: svn://svn.chromium.org/blink/trunk@168827 bbb929c8-8fbe-4397-9dbb-9b2b20218538