[web_tests] mark a test flaky

Bug: 974660, 976362
Change-Id: Iea8ad0cef870131b1b65df1bfc4b3b35a081baa8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1665895
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670285}

third_party/blink/web_tests/TestExpectations

@@ -6242,3 +6242,6 @@ crbug.com/973769 [ Linux ] media/video-pause-immediately.html [ Pass Failure ]
 crbug.com/973769 [ Linux ] media/video-played-ranges-1.html [ Pass Failure ]
 crbug.com/973769 [ Linux ] media/video-played-reset.html [ Pass Failure ]
 crbug.com/973769 [ Linux ] media/W3C/audio/events/event_timeupdate_manual.html [ Pass Failure ]
+
+# Flaky on Linux
+crbug.com/974660 [ Linux ] http/tests/devtools/elements/highlight/highlight-node-vertical-rl.js [ Pass Failure ]