Suppressing the flake in https://chromium-review.googlesource.com/974174 .

Two recent failures:

https://ci.chromium.org/buildbot/chromium.gpu.fyi/Mac%20FYI%20Experimental%20Release%20%28Intel%29/796
https://chromium-swarm.appspot.com/task?id=3c4df1de62e82b10&refresh=10&show_raw=1

and

https://ci.chromium.org/buildbot/chromium.gpu.fyi/Mac%20FYI%20Experimental%20Release%20%28Intel%29/801
https://chromium-swarm.appspot.com/task?id=3c4ff2fe528ef610&refresh=10&show_raw=1

Error message:

Error: GetUserMedia call failed with error name TexImage_TEXTURE_2D: reference value 255 differs from calculated: 0.03125 at index (row, column) 36084 (93, 93). TopLeft value:8, step:1.328125, flip_y:false

This test has been reported flaky on a few different platforms. It needs to be reexamined. Maybe there's a race condition in its startup where it attempts to read back the media stream before it's fully started.

As soon as the suppression has landed I'm going to downgrade this to P2.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/45501363aed7c21139d57031b1ce5dcebe8176e8

commit 45501363aed7c21139d57031b1ce5dcebe8176e8
Author: Kenneth Russell <kbr@chromium.org>
Date: Wed Mar 21 21:21:07 2018

Suppress depthStreamToRGBAUint8Texture flake on Mac 10.13 Intel.

TBR=gklassen@chromium.org, jmadill@chromium.org
NOTRY=true

Bug: 824438
Cq-Include-Trybots: luci.chromium.try:linux_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ib92b27fe4a418cbb570a565cf3f1213375f35290
Reviewed-on: https://chromium-review.googlesource.com/974174
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544849}
[modify] https://crrev.com/45501363aed7c21139d57031b1ce5dcebe8176e8/content/test/gpu/gpu_tests/depth_capture_expectations.py

Thanks kbr@. I'll re-check the logic - the error could be related to the test not covering border case, based on error message.