ash: Make UserActivityDetector ignore synthetic mouse events

This may have been preventing us from suspending (e.g.
mouse event is synthesized in response to lock window being
shown so Chrome tells powerd that the user is active).

BUG=133419
TEST=added


Review URL: https://chromiumcodereview.appspot.com/10574044

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@143437 0039d316-1c4b-4281-b951-d872f2087c98
  void reset_stats() { num_invocations_ = 0; }

  void reset_stats() { num_invocations_ = 0; }
