This makes the device type selection part of the ANGLE platform
extension. We currently support NULL driver selection on all
available back-ends (although on the NULL back-end, it already
has no device type). Optionally we could expose certain features
of this as separate extensions.

This currently also supports the old hidden enum, until we can
update Chrome and consolidate to the new exposed official enum.

Bug: angleproject:2159

Change-Id: I85d0811098e644e8192c207673af9e18ed7c1da2
Reviewed-on: https://chromium-review.googlesource.com/846021
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>