Suppress sub_dynamic WebGL2 flake

Suppresses a flake in WebGL2 conformance's
deqp/functional/gles3/shadermatrix/sub_dynamic.html test.

TBR=ynovikov@chromium.org

Bug: 1093482
Change-Id: I30c321811842293b7dc370fd23db506047059270
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2258675Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781067}

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

@@ -131,6 +131,8 @@ crbug.com/912579 [ opengl win passthrough intel ] conformance2/rendering/out-of-
 # Flakily times out on driver 26.20.100.8141.
 crbug.com/1093482 [ win10 intel-0x3e92 ] deqp/functional/gles3/shadermatrix/div_dynamic.html [ RetryOnFailure ]
 crbug.com/1093482 [ win10 intel-0x5912 ] deqp/functional/gles3/shadermatrix/div_dynamic.html [ RetryOnFailure ]
+crbug.com/1093482 [ win10 intel-0x3e92 ] deqp/functional/gles3/shadermatrix/sub_dynamic.html [ RetryOnFailure ]
+crbug.com/1093482 [ win10 intel-0x5912 ] deqp/functional/gles3/shadermatrix/sub_dynamic.html [ RetryOnFailure ]
 
 # Intel failed issues
 crbug.com/795030 [ opengl win passthrough intel ] deqp/functional/gles3/shadercommonfunction.html [ Failure ]