Use MessageLoopCurrent::IsSet in /components/policy

As stated in message_loop_current.h, |MessageLoopCurrent::IsSet()|
should be preferred to checking the return value of
|MessageLoopCurrent::Get()|.

Bug: 825327
Change-Id: I90bf74dd9929eeb946e8e2e3ee1ac2e9f093a938
Reviewed-on: https://chromium-review.googlesource.com/1025770Reviewed-by: Lutz Justen <ljusten@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553086}

components/policy/core/common/mock_configuration_policy_provider.cc

@@ -26,7 +26,7 @@ void MockConfigurationPolicyProvider::UpdateChromePolicy(
   bundle->Get(PolicyNamespace(POLICY_DOMAIN_CHROME, std::string()))
       .CopyFrom(policy);
   UpdatePolicy(std::move(bundle));
-  if (base::MessageLoopCurrent::Get())
+  if (base::MessageLoopCurrent::IsSet())
     base::RunLoop().RunUntilIdle();
 }