Fixes in UserGestureIndicator

This CL fixes problems discovered in https://codereview.chromium.org/235983016/ post landing.

BUG=163072

Review URL: https://codereview.chromium.org/239963005

git-svn-id: svn://svn.chromium.org/blink/trunk@172128 bbb929c8-8fbe-4397-9dbb-9b2b20218538