Generalize the main thread event queue into a common event queue.

Teach the WebInputEventQueue about ScopedWebInputEvents so we can
use a single queue instead of one per event class.

This reduces code duplication and allows for another patch to then take
advantage of this for all events.

BUG=624012,599152

Review-Url: https://codereview.chromium.org/2170913002
Cr-Commit-Position: refs/heads/master@{#407826}
7 files changed