Remove handlers for user gesture callbacks

This reverts the CL https://codereview.chromium.org/82693006.

That code was added as a support for password autofill value protection (Chromium part was https://codereview.chromium.org/83023017). That was recently refactored (https://codereview.chromium.org/163843002/) and does not need the Blink support any more.

Precise identification of the revert, as generated by GIT:
Revert "Adding handlers for user gesture callbacks. Whenever a UserGestureToken is created, calls all registered callbacks to alert them of gesture."
This reverts commit b011272eda3e5193a5966522b9f845dd40fae1bd.

BUG=337429,163072

Review URL: https://codereview.chromium.org/174893002

git-svn-id: svn://svn.chromium.org/blink/trunk@167683 bbb929c8-8fbe-4397-9dbb-9b2b20218538