Introduce the concept of RAF aligned input events to the MainThreadEventQueue.
Currently behind a feature flag and is disabled.

Short Design Doc:

https://docs.google.com/document/d/1aM9AqyYuceRHOmsJZXFKcgi_D4jvFAymYvI36OEwIpI/edit?usp=sharing

BUG=625693

Review-Url: https://codereview.chromium.org/2166703003
Cr-Commit-Position: refs/heads/master@{#415099}
18 files changed