Allow a list of fallback formats as well as a single one in the format
map.  The first supported format is used.  No functional change.

BUG=angleproject:2655

Change-Id: Ica312b7899471a7a65184a6921713b79da056f31
Reviewed-on: https://chromium-review.googlesource.com/1214847Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>