Mark as ChromeDriverTestLegacy.testContextMenuEventFired as flaky.

Bug: 1156576
Change-Id: I463f384ab10fa81d1a2888de85dd03374c2a1d0f
TBR: shengfa@google.com
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577367Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834666}

chrome/test/chromedriver/test/run_py_tests.py

@@ -104,6 +104,8 @@ _OS_SPECIFIC_FILTER['mac'] = [
     'MobileEmulationCapabilityTest.testTapElement',
     # https://bugs.chromium.org/p/chromium/issues/detail?id=1011225
     'ChromeDriverTest.testActionsMultiTouchPoint',
+    # Flaky: https://crbug.com/1156576.
+    'ChromeDriverTestLegacy.testContextMenuEventFired',
 ]
 
 _DESKTOP_NEGATIVE_FILTER = [