[sync] Fix E2E tests crashing with search engines

Starting with http://crrev.com/c/2434329, which actually landed some
time ago now, TwoClientSearchEnginesSyncTest::SetupClients() incorrectly
assumed that there always exist two clients.

The caveat is, for E2E tests, ResetSyncForPrimaryAccount() sets up
exactly one client as a convenient way to reset sync.

Change-Id: Ie7967694e13090a5317de4d2422ba2fadecd7aa9
Bug: 1147195
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527203Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825742}

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

@@ -41,10 +41,13 @@ class TwoClientSearchEnginesSyncTest : public SyncTest {
       return false;
     }
 
-    search_test_utils::WaitForTemplateURLServiceToLoad(
-        TemplateURLServiceFactory::GetForProfile(GetProfile(0)));
-    search_test_utils::WaitForTemplateURLServiceToLoad(
-        TemplateURLServiceFactory::GetForProfile(GetProfile(1)));
+    // In most cases this codepath should have exactly two clients, but there is
+    // an exception for E2E tests, where ResetSyncForPrimaryAccount()
+    // temporarily sets up one only.
+    for (int i = 0; i < num_clients(); ++i) {
+      search_test_utils::WaitForTemplateURLServiceToLoad(
+          TemplateURLServiceFactory::GetForProfile(GetProfile(i)));
+    }
 
     return true;
   }