Mark ScalePatching as flaky.

BUG=871799
TBR=miu@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I63e3694a2f006daab624cf434da682dbd725f789
Reviewed-on: https://chromium-review.googlesource.com/1165313
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581247}

components/viz/common/gl_helper_unittest.cc

@@ -1313,7 +1313,8 @@ TEST_P(GLHelperPixelReadbackTest, ScaleTest) {
   }
 }
 
-TEST_P(GLHelperPixelReadbackTest, ScalePatching) {
+// FLAKY: https://crbug.com/871799
+TEST_P(GLHelperPixelReadbackTest, FLAKY_ScalePatching) {
   for (int flipped_source = 0; flipped_source <= 1; ++flipped_source) {
     for (int pattern = 0; pattern < 3; ++pattern) {
       TestScalePatching(