Mark deqp/functional/gles3/shaderstruct.html as flaky on Mac/AMD

BUG=1005593
TEST=bot
TBR=kbr@chromium.org

Change-Id: I775dbee764c8dfb1327d72ad14b8a1d78b9c4e26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1812247Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697893}

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

@@ -460,6 +460,8 @@ crbug.com/663601 [ mac amd ] deqp/functional/gles3/textureshadow/* [ RetryOnFail
 crbug.com/679058 [ mac amd ] deqp/functional/gles3/texturespecification/teximage2d_unpack_params.html [ RetryOnFailure ]
 crbug.com/642822 [ mac amd ] conformance2/rendering/clipping-wide-points.html [ Failure ]
 
+crbug.com/1005593 [ mac amd ] deqp/functional/gles3/shaderstruct.html [ RetryOnFailure ]
+
 # Mac Intel
 crbug.com/886970 [ mac intel-0xa2e ] conformance/rendering/canvas-alpha-bug.html [ Failure ]
 crbug.com/678526 [ mac intel ] conformance2/rendering/framebuffer-texture-level1.html [ Failure ]