pixel_expectations.py: Suppress flaky Pixel_BackgroundImage test.

Supress Pixel_BackgroundImage test which is flaky on
Nexus 5X(android-marshmallow-arm64-rel trybot).

Bug: 883500
Change-Id: I54412764b19e8cea273f98594cc8c6ad2ba1507b
Reviewed-on: https://chromium-review.googlesource.com/c/1441312
Commit-Queue: vikas soni <vikassoni@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626925}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -117,6 +117,10 @@ class PixelExpectations(GpuTestExpectations):
     self.Fail('Pixel_BackgroundImage',
         ['android', ('qualcomm', 'Adreno (TM) 430')], bug=883500)
 
+    # Flakes on Nexus 5X.
+    self.Flaky('Pixel_BackgroundImage',
+        ['android', ('qualcomm', 'Adreno (TM) 418')], bug=883500)
+
     # TODO(wangxianzhu): Re-enable after and rebaselining
     self.Fail('Pixel_CSSFilterEffects', bug=836884)
     self.Fail('Pixel_CSSFilterEffects_NoOverlays', bug=836884)