Add TestExpectation for fast/overflow/clip-rects-fixed-ancestor.html

TBR=chrishtr@chromium.org

Bug: 921242
Change-Id: Ifa915bc86e5148d5df71a744ef6f41c00881115f
Reviewed-on: https://chromium-review.googlesource.com/c/1429881
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625146}
diff --git a/third_party/blink/web_tests/TestExpectations b/third_party/blink/web_tests/TestExpectations
index 18c2ea8..db3c1e0 100644
--- a/third_party/blink/web_tests/TestExpectations
+++ b/third_party/blink/web_tests/TestExpectations
@@ -5986,3 +5986,7 @@
 
 crbug.com/v8/8319 external/wpt/wasm/jsapi/memory/grow.any.html [ Pass Failure ]
 crbug.com/v8/8319 external/wpt/wasm/jsapi/memory/grow.any.worker.html [ Pass Failure ]
+
+# Sheriff 2019-01-22
+# Likely needs a rebaseline
+crbug.com/921242 [ Mac ] fast/overflow/clip-rects-fixed-ancestor.html [ Failure ]