Revert "Disable a crashing TwoClientBookmarksSyncTest."

This reverts commit 664491ff.

Reason for revert: the underlying issue is believed to be fixed after
https://chromium-review.googlesource.com/c/chromium/src/+/1029853

Original change's description:
> Disable a crashing TwoClientBookmarksSyncTest.
> 
> BUG=836426
> TBR=skym@chromium.org
> NOTRY=true
> 
> Change-Id: Ie9c07c1417ab693d517e1ac4ee3dc8082465a58a
> Reviewed-on: https://chromium-review.googlesource.com/1026790
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Commit-Queue: Lei Zhang <thestig@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#553299}

TBR=thestig@chromium.org,skym@chromium.org

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

Bug: 836426
Change-Id: I53b2cac1ee6720cbbbd9d2650292cac6cc551bfe
Reviewed-on: https://chromium-review.googlesource.com/1032411Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554350}

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

@@ -1815,10 +1815,8 @@ IN_PROC_BROWSER_TEST_F(TwoClientBookmarksSyncTest,
   ASSERT_TRUE(BookmarksMatchVerifierChecker().Wait());
 }
 
-// Test is crashing. https://crbug.com/836426
-IN_PROC_BROWSER_TEST_F(
-    TwoClientBookmarksSyncTest,
-    DISABLED_FirstClientEnablesEncryptionWithPassSecondChanges) {
+IN_PROC_BROWSER_TEST_F(TwoClientBookmarksSyncTest,
+                       FirstClientEnablesEncryptionWithPassSecondChanges) {
   ASSERT_TRUE(SetupSync()) << "SetupSync() failed.";
   ASSERT_TRUE(AllModelsMatchVerifier());