Add autotest private API for taking a screenshot

This is a temporary solution to taking screenshots for tast-tests that
will work properly on ARM devices as well as deal with hardware
overlays. Longer term the plan is to implement this via the DRM API with
a write-back connector and then use that from the CLI screenshot
program.

BUG=chromium:849438,chromium:880597,chromium:887016
TEST=tast run graphics.Screenshot (after updating it to use this)

Change-Id: I3919822599338e4786c715d7b0419742417b83c2
Reviewed-on: https://chromium-review.googlesource.com/1247181
Commit-Queue: Jeffrey Kardatzke <[email protected]>
Reviewed-by: James Cook <[email protected]>
Reviewed-by: Ben Wells <[email protected]>
Reviewed-by: Achuith Bhandarkar <[email protected]>
Cr-Commit-Position: refs/heads/master@{#595173}
7 files changed