Reland "Reenable a test in TwoClientPreferencesSyncTest on CrOS"

This is a reland of 05048d0f

Meanwhile, a patch has landed that is believed to fix the issue
observed in recent flakes.

Original change's description:
> Reenable a test in TwoClientPreferencesSyncTest on CrOS
>
> Recent changes (linked to the bug) are believed to have deflaked the
> test, so let's reenable it on all platforms.
>
> Bug: 873902
> Change-Id: I9804fa4d83c3b1afb9b71356d134736b63b11298
> Reviewed-on: https://chromium-review.googlesource.com/c/1343083
> Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#609649}

Bug: 873902
Change-Id: I0c82df1ed8ca63ec4c0490a3f76ac1c9efddd5c6
Reviewed-on: https://chromium-review.googlesource.com/c/1347351Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610196}

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

@@ -8,7 +8,6 @@
 #include "base/macros.h"
 #include "base/strings/stringprintf.h"
 #include "base/test/metrics/histogram_tester.h"
-#include "build/build_config.h"
 #include "chrome/browser/sync/test/integration/feature_toggler.h"
 #include "chrome/browser/sync/test/integration/preferences_helper.h"
 #include "chrome/browser/sync/test/integration/profile_sync_service_harness.h"
@@ -140,17 +139,8 @@ IN_PROC_BROWSER_TEST_P(TwoClientPreferencesSyncTest,
   ASSERT_TRUE(ListPrefMatchChecker(prefs::kURLsToRestoreOnStartup).Wait());
 }
 
-// Disabled due to flakiness on Chrome OS: https://crbug.com/873902.
-#if defined(OS_CHROMEOS)
-#define MAYBE_SingleClientEnabledEncryptionBothChanged \
-  DISABLED_SingleClientEnabledEncryptionBothChanged
-#else
-#define MAYBE_SingleClientEnabledEncryptionBothChanged \
-  SingleClientEnabledEncryptionBothChanged
-#endif
-IN_PROC_BROWSER_TEST_P(
-    TwoClientPreferencesSyncTest,
-    E2E_ENABLED(MAYBE_SingleClientEnabledEncryptionBothChanged)) {
+IN_PROC_BROWSER_TEST_P(TwoClientPreferencesSyncTest,
+                       E2E_ENABLED(SingleClientEnabledEncryptionBothChanged)) {
   ASSERT_TRUE(SetupSync());
   ASSERT_TRUE(BooleanPrefMatchChecker(prefs::kHomePageIsNewTabPage).Wait());
   ASSERT_TRUE(StringPrefMatchChecker(prefs::kHomePage).Wait());