[Flaky][Mac] Disable flaky DownloadShelfControllerTest test

Disabling AutoCloseAfterOpenWithMouseOffShelf which wasn't for some
reason disabled yet although it's similar flaky as all others.

Latest recorded failure:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests%20%28dbg%29/4744

The flakiness dashboard supports this action:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#showAllRuns=true&testType=unit_tests&tests=DownloadShelfControllerTest.AutoCloseAfterOpenWithMouseOffShelf

TBR=sdy@chromium.org,benwells@chromium.org

Bug: 849477
Change-Id: Ic793f6f0139c5d56148383caf9ff2890cdfe2420
Reviewed-on: https://chromium-review.googlesource.com/1179674Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584013}

chrome/browser/ui/cocoa/download/download_shelf_controller_unittest.mm

@@ -271,8 +271,10 @@ TEST_F(DownloadShelfControllerTest,
   EXPECT_EQ(1, shelf_.get()->cancelAutoCloseCount_);
 }
 
+// TODO(crbug.com/849477) Disabled for flakiness.
 // Test of autoclosing behavior after opening a download item.
-TEST_F(DownloadShelfControllerTest, AutoCloseAfterOpenWithMouseOffShelf) {
+TEST_F(DownloadShelfControllerTest,
+       DISABLED_AutoCloseAfterOpenWithMouseOffShelf) {
   base::scoped_nsobject<DownloadItemController> item(CreateItemController());
   [shelf_ showDownloadShelf:YES isUserAction:NO animate:YES];
   EXPECT_TRUE([shelf_ isVisible]);