disable http/tests/devtools/layers/layer-compositing-reasons.js

It is flaky

TBR=caseq@chromium.org

Bug: 794180
No-Try: True
Change-Id: Id66f1df0844ba8f6bed757b244c8c50d65eb48d4
Reviewed-on: https://chromium-review.googlesource.com/822416Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523444}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3676,3 +3676,6 @@ crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/dou
 crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/doubletap-to-jump-forwards-too-short.html [ Skip ]
 crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/doubletap-on-play-button.html [ Skip ]
 crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/doubletap-to-toggle-fullscreen.html [ Skip ]
+
+# Sheriff faulures 2017-12-12
+crbug.com/794180 http/tests/devtools/layers/layer-compositing-reasons.js [ Failure Pass ]