Check that only a single delegate for the Android PolicyProvider exists.

BUG=484997

Review URL: https://codereview.chromium.org/1129323005

Cr-Commit-Position: refs/heads/master@{#329378}

components/policy/core/common/policy_provider_android.cc

@@ -32,6 +32,7 @@ void PolicyProviderAndroid::SetShouldWaitForPolicy(
 
 void PolicyProviderAndroid::SetDelegate(
     PolicyProviderAndroidDelegate* delegate) {
+  DCHECK(!delegate || !delegate_);
   delegate_ = delegate;
 }