[web tests] Declare Mac10.14 in TestExpectations

Add the Mac10.14 to a tag declaration list in TestExpectations.

Bug: 986447
Bug: 1028242
Change-Id: I74371a2e6f34573259546697502de6c89001e76d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031712
Commit-Queue: Rakib Hasan <rmhasan@google.com>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737079}

third_party/blink/web_tests/TestExpectations

 # tags: [ Android Fuchsia IOS IOS12.2 IOS13.0 Linux Mac Mac10.10 Mac10.11 Mac10.12 Retina
-#         Mac10.13 Win Win7 Win10 ]
+#         Mac10.13 Mac10.14 Win Win7 Win10 ]
 # tags: [ Release Debug ]
 # results: [ Timeout Crash Pass Failure Slow Skip ]