Viz suppression for ThumbnailTest.ShouldCaptureOnNavigatingAwaySlowPageLoad

TBR=treib@chromium.org

Bug: 843921
Change-Id: I49f76869bbc7cd0e8abd5ac889ddec5e0ac1282d
No-Tree-Checks: True
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/1063526
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559462}

testing/buildbot/filters/viz.browser_tests.filter

@@ -33,3 +33,6 @@
 
 # Flaking with missing browser. https://crbug.com/842664
 -IndependentOTRProfileManagerTest.DeleteImmediatelyWhenBrowsersAlreadyClosed
+
+# https://crbug.com/843921
+-ThumbnailTest.ShouldCaptureOnNavigatingAwaySlowPageLoad