Enable SingleClientSessionsSyncTest.NavigationChainAlteredDestructively

It was disabled for flakiness ~6 months ago; let's see if that is still
accurate. (Likely it is, but at least the flakes don't repro locally.)

Bug: 1043899
Change-Id: Iaad54efee9a2548cca72c33adb6c5160b8b2d19d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2338734
Commit-Queue: Victor Vianna <victorvianna@google.com>
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/master@{#794977}

chrome/browser/sync/test/integration/single_client_sessions_sync_test.cc

@@ -591,10 +591,8 @@ IN_PROC_BROWSER_TEST_F(SingleClientSessionsSyncTest,
   ExpectNavigationChain({first_url, second_url});
 }
 
-// Flaky for reasons that likely have nothing to do with the test itself. See
-// crbug.com/1043899 and crbug.com/992207.
 IN_PROC_BROWSER_TEST_F(SingleClientSessionsSyncTest,
-                       DISABLED_NavigationChainAlteredDestructively) {
+                       NavigationChainAlteredDestructively) {
   ASSERT_TRUE(SetupSync()) << "SetupSync() failed.";
   ASSERT_TRUE(CheckInitialState(0));