[ios] Fix typo in MockConfigurationPolicyProvider.

BUG=1024115

Change-Id: I1702b5f91f9b3969e7aafdacbcfab1236c91ad64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2119217Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753270}

components/policy/core/common/mock_configuration_policy_provider.cc

@@ -28,7 +28,7 @@ void MockConfigurationPolicyProvider::UpdateChromePolicy(
       .CopyFrom(policy);
   UpdatePolicy(std::move(bundle));
   bool spin_run_loop = base::MessageLoopCurrent::IsSet();
-#if defined(IS_IOS)
+#if defined(OS_IOS)
   // On iOS, the UI message loop does not support RunUntilIdle().
   spin_run_loop &= !base::MessageLoopCurrentForUI::IsSet();
 #endif  // defined(OS_IOS)