Mark http/tests/devtools/profiler/live-line-level-heap-profile.js flaky

TBR=gayane@chromium.org
NOTRY=true

Bug: 971262
Change-Id: I0a5a2b0ea5cb6d046b0efbdbb4c67637afb015aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645130Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666295}

third_party/blink/web_tests/TestExpectations

@@ -5719,3 +5719,4 @@ crbug.com/970142 virtual/blink-cors/http/tests/security/mixedContent/insecure-cs
 crbug.com/971031 [ Mac ] fast/dom/timer-throttling-hidden-page.html [ Pass Failure ]
 crbug.com/971252 [ Mac ] external/wpt/media-source/mediasource-play.html [ Pass Timeout ]
 crbug.com/971259 media/controls/volumechange-stopimmediatepropagation.html [ Pass Failure ]
+crbug.com/971262 http/tests/devtools/profiler/live-line-level-heap-profile.js [ Pass Timeout ]