sheriff: Disable element-request-fullscreen-and-move-to-iframe on macOS 10.13

Has been flaky on macOS 10.13:

https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests/25584

https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests/25587

https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests/25588

TBR=foolip@chromium.org

Bug: 1007228
Change-Id: I2cc599f67cba5e4a8768a24891f69563433e7fbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227942Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774534}

third_party/blink/web_tests/TestExpectations

@@ -5877,3 +5877,6 @@ crbug.com/1088007 virtual/gpu/fast/canvas/OffscreenCanvas-zero-size-readback.htm
 crbug.com/1088007 virtual/gpu/fast/canvas/OffscreenCanvasClearRect.html [ Pass Failure ]
 crbug.com/1088007 virtual/gpu/fast/canvas/OffscreenCanvasClearRectPartial.html [ Pass Failure ]
 crbug.com/1088007 virtual/gpu/fast/canvas/OffscreenCanvas-MessageChannel-transfer.html [ Pass Failure ]
+
+# Sheriff 2020-06-03
+crbug.com/1007228 [ Mac10.13 ] external/wpt/fullscreen/api/element-request-fullscreen-and-move-to-iframe-manual.html [ Pass Failure ]