[Sheriff] Disable memory_desktop.browse:media:imgur

Temporarily disable the test to clear the bot errors, because the
test is failing frequently/flakily due to crbug.com/749773.

TBR=nednguyen@chromium.org, adithyas@chromium.org, yutak@chromium.org
NOTRY=true

Bug: 749773
Change-Id: I6c4540ef2a357b47c4c9dc00ced499196bc7a566
Reviewed-on: https://chromium-review.googlesource.com/595254
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490835}

tools/perf/benchmarks/system_health_smoke_test.py

@@ -71,6 +71,7 @@ _DISABLED_TESTS = frozenset({
   # crbug.com/749773 crbug.com/749918
   'benchmarks.system_health_smoke_test.SystemHealthBenchmarkSmokeTest.system_health.memory_mobile.load:news:cnn', # pylint: disable=line-too-long
   'benchmarks.system_health_smoke_test.SystemHealthBenchmarkSmokeTest.system_health.memory_desktop.browse:news:hackernews', # pylint: disable=line-too-long
+  'benchmarks.system_health_smoke_test.SystemHealthBenchmarkSmokeTest.system_health.memory_desktop.browse:media:imgur', # pylint: disable=line-too-long
 })