[ios] Disable UKM tests.

Bug: 853992
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Id441f479a233d0aa1cd6c2761da40819e0be2bc2
Reviewed-on: https://chromium-review.googlesource.com/1105998
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568427}

ios/chrome/browser/metrics/ukm_egtest.mm

@@ -252,6 +252,13 @@ void SignOut() {
 
 @implementation UKMTestCase
 
+// Per crbug.com/853992, Entire test suite is failing regularly.
++ (NSArray*)testInvocations {
+  if (IsIPadIdiom())
+    return @[];
+  return [super testInvocations];
+}
+
 + (void)setUp {
   [super setUp];
   if (!base::FeatureList::IsEnabled(ukm::kUkmFeature)) {