Disabling failing sync EG test

This CL disable a sync EG test failing almost consistently on iPhone
simulator iOS 10 and which is flaky on other configuration.
As all the other tests of this test suite are disabled, I am disabling
it instead of marking it as flaky, as there is probably something wrong
with the whole test suite.

Bug: 821490
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I5bada13c540bc3d95c2082109d3e9299b873cc39
Reviewed-on: https://chromium-review.googlesource.com/1054233Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558221}

ios/chrome/browser/ui/settings/sync_utils/sync_fake_server_egtest.mm

@@ -178,7 +178,8 @@ void AssertNumberOfEntitiesWithName(int entity_count,
 }
 
 // Tests that the local cache guid does not change when sync is restarted.
-- (void)testSyncCheckSameCacheGuid_SyncRestarted {
+// TODO(crbug.com/821490): Reenable the test.
+- (void)DISABLED_testSyncCheckSameCacheGuid_SyncRestarted {
   // Sign in the fake identity.
   ChromeIdentity* identity = [SigninEarlGreyUtils fakeIdentity1];
   ios::FakeChromeIdentityService::GetInstanceFromChromeProvider()->AddIdentity(