Add a new "emulatedEtc1" limitation.

Bug: chromium:1048244
Change-Id: Ia04746773db3ad75781a8adfda988d36bd2c57f7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2718865Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>