Run WebGL 2.0 tests on ANGLE's Metal backend.

Deploys this test suite on the following:

ANGLE CI and CQ bots:
  mac-angle-chromium-amd
  mac-angle-chromium-intel

chromium.gpu.fyi bots:
  Mac FYI ASAN (Intel)
  Mac FYI Release (Apple M1)
  Mac FYI Release (Intel)
  Mac FYI Retina ASAN (AMD)
  Mac FYI Retina Release (AMD)

Optional GPU trybots:
  Optional Mac Release (Intel)
  Optional Mac Retina Release (AMD)

Verified via:
  ./testing/buildbot/generate_buildbot_json.py --query \
    test/'webgl2_conformance_metal_passthrough_tests'/bots

Suppress failures seen across GPUs, and on AMD and Intel
specifically, under ANGLE bug 6430.

Thanks to ynovikov@ for the first draft of this CL.

Bug: 1245398
Bug: angleproject:6430
Change-Id: Ib6b0a4f5e9b8500843f032e42803e22184fbcb34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3254282
Reviewed-by: Brian Sheedy <[email protected]>
Commit-Queue: Kenneth Russell <[email protected]>
Cr-Commit-Position: refs/heads/main@{#939015}
diff --git a/testing/buildbot/test_suite_exceptions.pyl b/testing/buildbot/test_suite_exceptions.pyl
index a04af47..f108214 100644
--- a/testing/buildbot/test_suite_exceptions.pyl
+++ b/testing/buildbot/test_suite_exceptions.pyl
@@ -3200,6 +3200,12 @@
       },
     },
   },
+  'webgl2_conformance_metal_passthrough_tests': {
+    'remove_from': [
+      # Not enough capacity.
+      'Mac FYI Retina Release (NVIDIA)',
+    ],
+  },
   'webgl2_conformance_validating_tests': {
     'remove_from': [
       # The Mac NVIDIA Retina bots don't have the capacity to run