This was broken by the angle test platforms refactor patch,
https://chromium-review.googlesource.com/#/c/276460/. It was
leading to the wrong tests be executed for the perf tests, and
also had bugs with supporting a default platform.

BUG=None
TEST=angle_perftests

Change-Id: Ib99342d4189fd98bf93dce088d15afcecc4a7e37
Reviewed-on: https://chromium-review.googlesource.com/277036Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>