[Sheriff] mark bfast/borders/border-radius-mask-* tests as failure on Mac.

E.g. https://ci.chromium.org/p/chromium/builders/ci/Mac10.11%20Tests/40811
Unexpected Failures:
* fast/borders/border-radius-mask-canvas-all.html
* fast/borders/border-radius-mask-canvas-border.html
* fast/borders/border-radius-mask-canvas-padding.html
* fast/borders/border-radius-mask-canvas-with-mask.html
* fast/borders/border-radius-mask-canvas-with-shadow.html
* fast/borders/border-radius-mask-canvas.html
* fast/borders/border-radius-mask-video-ratio.html
* fast/borders/border-radius-mask-video-shadow.html
* fast/borders/border-radius-mask-video.html

Bug: 979422
Change-Id: I77650a186cf4dcb029b58b0d839aab07ee40688a
TBR: chrishtr@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1727096Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#682594}

third_party/blink/web_tests/TestExpectations

@@ -6431,3 +6431,14 @@ crbug.com/626703 [ Mac ] external/wpt/css/cssom-view/scrollIntoView-sideways-rl-
 crbug.com/988890 [ Win7 Release ] external/wpt/referrer-policy/strict-origin-when-cross-origin/attr-referrer/same-origin/http-http/img-tag/keep-origin-redirect/same-insecure.http.html [ Pass Timeout ]
 crbug.com/988890 [ Win7 Release ] virtual/blink-cors/external/wpt/referrer-policy/strict-origin-when-cross-origin/attr-referrer/same-origin/http-http/img-tag/keep-origin-redirect/same-insecure.http.html [ Pass Timeout ]
 crbug.com/988890 [ Win7 Release ] virtual/not-omt-sw-fetch/external/wpt/referrer-policy/strict-origin-when-cross-origin/attr-referrer/same-origin/http-http/img-tag/keep-origin-redirect/same-insecure.http.html [ Pass Timeout ]
+
+# Sheriff 2019-07-31
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-video-shadow.html [ Failure ]
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-canvas-with-shadow.html [ Failure ]
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-canvas-all.html [ Failure ]
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-canvas-padding.html [ Failure ]
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-canvas.html [ Failure ]
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-video-ratio.html [ Failure ]
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-canvas-with-mask.html [ Failure ]
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-canvas-border.html [ Failure ]
+crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-video.html [ Failure ]