Disable Flaky ForegroundedOrClosedTest.SingleTab test

ForegroundedOrClosedTest.SingleTab has started to flake on ChromeOS. This
disables the test until the cause can be found.

TEST=ForegroundedOrClosedTest.SingleTab
TBR=fdoray@chromium.org

Bug: 839886
Change-Id: I650bc975a9e4795218eb205a628d3c604f7fcdcf
Reviewed-on: https://chromium-review.googlesource.com/1044533Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556112}

chrome/browser/resource_coordinator/tab_activity_watcher_unittest.cc

@@ -581,8 +581,14 @@ class ForegroundedOrClosedTest : public TabActivityWatcherTest {
   DISALLOW_COPY_AND_ASSIGN(ForegroundedOrClosedTest);
 };
 
+// TODO(michaelpg): test is flaky on ChromeOS. https://crbug.com/839886
+#if defined(OS_CHROMEOS)
+#define MAYBE_SingleTab DISABLED_SingleTab
+#else
+#define MAYBE_SingleTab SingleTab
+#endif
 // Tests TabManager.Backgrounded.ForegroundedOrClosed UKM logging.
-TEST_F(ForegroundedOrClosedTest, SingleTab) {
+TEST_F(ForegroundedOrClosedTest, MAYBE_SingleTab) {
   Browser::CreateParams params(profile(), true);
   std::unique_ptr<Browser> browser =
       CreateBrowserWithTestWindowForParams(&params);