input: Dispatch synthesized events at regular intervals.

Instead of dispatching events at begin-frame + offset, dispatch events
at regular intervals (~16ms). The timer starts at an offset from the
first begin frame.

BUG=722921

Review-Url: https://codereview.chromium.org/2886263002
Cr-Commit-Position: refs/heads/master@{#473922}
3 files changed