Document LayoutTests baseline fallback and optimization

Bug: 767356
Change-Id: I2f8df4e507c4de83ac4c5166870cf673d1bd8465
Reviewed-on: https://chromium-review.googlesource.com/755376
Commit-Queue: Robert Ma <robertma@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516350}
3 files changed