Implemented smooth scrolling using xinput2 in X11.

Adds support for Xinput 2.1 smooth scrolling for hardware that supports it
(such as touchpads and some mice). This provides similar behaviour to that seen
on Mac OS X.

BUG=384970

Review URL: https://codereview.chromium.org/688253002

Cr-Commit-Position: refs/heads/master@{#368645}
8 files changed