WebView: speed up user gesture notification for GestureTap

Currently, when about to return, WebViewImpl::handleInputEvent notifies
Chromium if user gesture has been registered during event processing.

This notification comes too late when the first input event causes
a form submission, as explained in the code comment, and also in
http://crbug.com/372595#c13 .

This CL adds a special case handling for the only event with this problem:
GestureTap, so that this event is reported as a user gesture even before any
event handling takes place. This implements solution 1) from
http://crbug.com/372595#c14 .

BUG=372595

Review URL: https://codereview.chromium.org/288423002

git-svn-id: svn://svn.chromium.org/blink/trunk@174337 bbb929c8-8fbe-4397-9dbb-9b2b20218538