Disable failing webgl_conformance test on CrOS.

Failing consistently after crrev.com/c/1598921 (See bug.)

Disabling for now.

TBR=kbr@chromium.org

Bug: 977830
Change-Id: Ib5ee0ce7e67ff6def2908d368f3885c0c5c02812
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674326
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671777}

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

@@ -591,4 +591,5 @@ crbug.com/957807 [ chromeos ] conformance/rendering/clipping-wide-points.html [
 crbug.com/957807 [ chromeos ] conformance/rendering/line-rendering-quality.html [ Skip ]
 crbug.com/957807 [ chromeos ] conformance/uniforms/uniform-samplers-test.html [ Skip ]
 crbug.com/957807 [ chromeos ] deqp/data/gles2/shaders/conversions.html [ Skip ]
+crbug.com/977830 [ chromeos ] deqp/data/gles2/shaders/functions.html [ Failure ]
 crbug.com/957807 [ chromeos ] deqp/data/gles2/shaders/swizzles.html [ Skip ]