This fixes the sharding to use the specified back-end. With
the bug ANGLE would start the platform default back-end instead.

Bug: angleproject:3162
Change-Id: Ib2453dd3c58ea40fb36619301865ae0818038d15
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2447043
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>