Disable flaky AppServiceImplTest.PreferredApps test

TBR=mxcai@chromium.org, dominickn@chromium.org

Bug: 1074596
Change-Id: I12e457f366b41b605684290e89ad3f9b149b016d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2167443Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762854}

chrome/services/app_service/app_service_impl_unittest.cc

@@ -262,7 +262,8 @@ TEST_F(AppServiceImplTest, PubSub) {
   }
 }
 
-TEST_F(AppServiceImplTest, PreferredApps) {
+// TODO(https://crbug.com/1074596) disabled due to flakiness
+TEST_F(AppServiceImplTest, DISABLED_PreferredApps) {
   // Test Initialize.
   AppServiceImpl::RegisterProfilePrefs(pref_service_.registry());
   ASSERT_TRUE(temp_dir_.CreateUniqueTempDir());