Fix small issues in new synthetic gestures implementation.

This is a small follow-up patch to http://crrev.com/27232002.
It fix some minor issues that were overlooked in the previous CL.

Add unit test for the smooth scroll gesture using mouse input.
Add instructions on how to implement a new synthetic gesture class.

BUG=297980

Review URL: https://codereview.chromium.org/50603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232125 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232125