Disable browse_accessibility:tools:gmail_compose

It has been very flaky. Please re-enable it with fix

TBR=nednguyen@chromium.org
NOTRY=true

Bug: 769809
Change-Id: I7077fbfa752ca5c046ae63475d974896f42e1225
Reviewed-on: https://chromium-review.googlesource.com/690515Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505071}

tools/perf/benchmarks/system_health_smoke_test.py

@@ -48,6 +48,9 @@ _DISABLED_TESTS = frozenset({
   # crbug.com/769263
   'benchmarks.system_health_smoke_test.SystemHealthBenchmarkSmokeTest.system_health.memory_desktop.play:media:soundcloud',  # pylint: disable=line-too-long
 
+  # crbug.com/769809
+  'benchmarks.system_health_smoke_test.SystemHealthBenchmarkSmokeTest.system_health.memory_desktop.browse_accessibility:tools:gmail_compose', # pylint: disable=line-too-long
+
   # crbug.com/
   'benchmarks.system_health_smoke_test.SystemHealthBenchmarkSmokeTest.system_health.memory_desktop.browse:news:nytimes',  # pylint: disable=line-too-long