gpu: Suppress failing WebGL conformance test on mac AMD.

TBR=kbr@chromium.org

Bug: 1037958
Change-Id: If8cc58b0bacc1ed5b50dfd32c2b3638acf17d589
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1982176Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727522}

content/test/gpu/gpu_tests/test_expectations/webgl2_conformance_expectations.txt

@@ -764,6 +764,8 @@ crbug.com/angleproject/3685 [ android opengles ] conformance2/transform_feedback
 crbug.com/angleproject/3686 [ android opengles ] deqp/functional/gles3/multisample.html [ Failure ]
 crbug.com/angleproject/3753 [ android opengles ] conformance/textures/misc/texture-upload-size.html [ Failure ]
 
+crbug.com/1037958 [ mac amd ] conformance2/transform_feedback/switching-objects.html [ RetryOnFailure ]
+
 # Conflicting expectations to test that the
 # "Expectations have no collisions" unittest works.