perf: Disable browse_accessibility:tools:gmail_compose due to flake

BUG=769809

Change-Id: Ie4f3f0a26c911948b7b8cda1fbe8e8fde2e3d8b0
Reviewed-on: https://chromium-review.googlesource.com/692774Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505394}

tools/perf/page_sets/system_health/expectations.py

@@ -26,6 +26,8 @@ class SystemHealthDesktopCommonExpectations(expectations.StoryExpectations):
                       [expectations.MAC_10_11], 'crbug.com/760966')
     self.DisableStory('browse:news:cnn',
                       [expectations.ALL_MAC], 'crbug.com/728576')
+    self.DisableStory('browse_accessibility:tools:gmail_compose',
+                      [expectations.ALL], 'crbug.com/769809')
 
 
 class SystemHealthDesktopMemoryExpectations(expectations.StoryExpectations):