Mark http/tests/devtools/resource-tree/resource-tree-htmlimports.js flaky on Mac

See linked bug for details.

TBR=lushnikov@chromium.org
NOTRY=true

Bug: 937416
Change-Id: If8c917fa804d9def71a5bf5ec24da85ae02fd840
Reviewed-on: https://chromium-review.googlesource.com/c/1497127Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636909}

third_party/blink/web_tests/TestExpectations

@@ -5964,3 +5964,4 @@ crbug.com/893659 [ Mac ] virtual/threaded/fast/events/pointerevents/pinch/pointe
 crbug.com/937358 [ Win ] virtual/scroll_customization/fast/events/touch/touch-handler-iframe-plugin-assert.html [ Pass Failure ]
 crbug.com/937378 [ Win7 ] http/tests/devtools/service-workers/service-workers-force-update-on-page-load.js [ Pass Timeout ]
 crbug.com/864994 [ Linux Win7 ] external/wpt/encoding/legacy-mb-korean/euc-kr/euckr-decode-ksc_5601.html [ Timeout Failure Pass ]
+crbug.com/937416 [ Mac ] http/tests/devtools/resource-tree/resource-tree-htmlimports.js [ Pass Failure ]