WebGL: Mark shader operator sequence test as flaky on Mac Intel

TBR=kbr@chromium.org

Bug: 1026588
Change-Id: I6563d489eaa3ea91879e0810f8373b721e1fa75b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036541Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738270}

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

@@ -488,6 +488,7 @@ crbug.com/606074 [ mac intel ] deqp/functional/gles3/texturefiltering/3d_combina
 crbug.com/658724 [ mac intel ] deqp/functional/gles3/framebufferblit/rect_03.html [ Failure ]
 crbug.com/658724 [ mac intel ] deqp/functional/gles3/framebufferblit/rect_04.html [ Failure ]
 crbug.com/658657 [ mac intel ] deqp/functional/gles3/texturespecification/random_teximage2d_2d.html [ Failure ]
+crbug.com/1026588 [ mac intel ] deqp/functional/gles3/shaderoperator/sequence.html [ RetryOnFailure ]
 crbug.com/905004 [ mac intel ] deqp/functional/gles3/shadertexturefunction/texturelod.html [ Failure ]
 crbug.com/905004 [ mac intel ] deqp/functional/gles3/shadertexturefunction/texturegrad.html [ Failure ]
 crbug.com/905004 [ mac intel ] deqp/functional/gles3/shadertexturefunction/textureprojgrad.html [ Failure ]