[Sheriff] Blink web tests: Skip failing tests on Mac.

Skip the following tests on Mac as they seem to be failing.

- http/tests/devtools/tracing/timeline-js/timeline-script-id.js
- virtual/threaded/http/tests/devtools/tracing/timeline-misc/timeline-event-causes.js

TBR=aerotwist@chromium.org
BUG=1107572

Change-Id: Idd6fa34188bf5efc5acb84f9bc5b8febbecd68e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310020Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790730}

third_party/blink/web_tests/TestExpectations

@@ -6917,3 +6917,6 @@ crbug.com/1107634 [ Mac10.10 ] virtual/text-antialias/selection/select-ligature-
 # Sheriff 2020-07-21
 crbug.com/1107923 [ Mac10.15 ] inspector-protocol/debugger/wasm-streaming-url.js [ Pass Failure ]
 
+# Sheriff 2020-07-22
+crbug.com/1107722 [ Mac ] http/tests/devtools/tracing/timeline-js/timeline-script-id.js [ Pass Failure ]
+crbug.com/1107722 [ Mac ] virtual/threaded/http/tests/devtools/tracing/timeline-misc/timeline-event-causes.js [ Pass Failure ]