GLImageIOSurface: enable BGRX GLImageBindTest

This CL extends gl_image_io_surface_unittest.cc to run
GLImageBindTest with BGRX, which is treated internally as
BGRA, by extending the value-to-compare-to and writing a
significant comment to explain the situation.

Bug: 803473
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ia9cfc20bb1be054da10cd0bd2526bc3f7f66d144
Reviewed-on: https://chromium-review.googlesource.com/876222
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530721}
2 files changed