Enable ForegroundedOrClosedTest.SingleTab test

This reverts commit 65c948ac.

Reason for revert: The cause of flakiness was fixed in
https://crrev.com/c/1092851.

Original change's description:
> 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/1044533
> Reviewed-by: Jonathan Ross <jonross@chromium.org>
> Commit-Queue: Jonathan Ross <jonross@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#556112}

TBR=fdoray@chromium.org,jonross@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 839886
Change-Id: I946125acee4ec518f25d6c1992ff3b3ecf9f6b0e
Reviewed-on: https://chromium-review.googlesource.com/1122797Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571912}

chrome/browser/resource_coordinator/tab_activity_watcher_unittest.cc

@@ -582,14 +582,8 @@ 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, MAYBE_SingleTab) {
+TEST_F(ForegroundedOrClosedTest, SingleTab) {
   Browser::CreateParams params(profile(), true);
   std::unique_ptr<Browser> browser =
       CreateBrowserWithTestWindowForParams(&params);