Mark editing/caret/caret-painting-low-dpi.html as flaky.

Bug: 892772
Change-Id: Id0359aeda04a4d4fbe41d3b7b6fb6df8897da60f
Tbr: schenney@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/1265816Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597325}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5248,3 +5248,4 @@ crbug.com/888609 [ Mac ] http/tests/devtools/coverage/gutter-css.js [ Pass Timeo
 # Sheriff 2018-10-03
 crbug.com/891530 [ Linux ] virtual/android/rootscroller/remove-rootscroller-crash.html [ Pass Timeout ]
 crbug.com/891510 [ Win7 ] fast/dom/inline-event-attributes-release.html [ Failure Pass ]
+crbug.com/892772 [ Mac ] editing/caret/caret-painting-low-dpi.html [ Failure Pass ]
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