Disable flaky ChromeVisibilityObserverBrowserTest.VisibilityTest

There are no obvious recent changes to have turned this test flaky. The
best hypothesis that I have is that something in the changes in
https://codereview.chromium.org/2939943004 was problematic, although
in that case it's unclear why it would only start turning up now.

TBR=asvitkine@chromium.org

Bug: 817172
Change-Id: Ic0665e7d8d606b5ecb01d11f641bcfe2ec82a1ab
Reviewed-on: https://chromium-review.googlesource.com/941121Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539784}

chrome/browser/metrics/desktop_session_duration/chrome_visibility_observer_browsertest.cc

@@ -31,7 +31,9 @@ class MockChromeVisibilityObserver : public metrics::ChromeVisibilityObserver {
 
 class ChromeVisibilityObserverBrowserTest : public InProcessBrowserTest {};
 
-IN_PROC_BROWSER_TEST_F(ChromeVisibilityObserverBrowserTest, VisibilityTest) {
+// TODO(https://crbug.com/817172): This test is flaky.
+IN_PROC_BROWSER_TEST_F(ChromeVisibilityObserverBrowserTest,
+                       DISABLED_VisibilityTest) {
   // Deactivate the initial browser window, to make observer start inactive.
   browser()->window()->Deactivate();
   base::RunLoop().RunUntilIdle();