[iOS Policy] Add comments about BrowserStatePolicyConnector lifecycle

Adds some comments clarifying why BrowserStatePolicyConnector isn't a
keyed service. Comments are added in the impl class, the mock class, and
the parent object (both the mock and the real application context).

Change-Id: I3a48a3ac903a3b37c916275ae18b78e77edf691f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2499961Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#821306}

ios/chrome/browser/application_context_impl.h

@@ -100,7 +100,10 @@ class ApplicationContextImpl : public ApplicationContext {
   std::unique_ptr<BreadcrumbPersistentStorageManager>
       breadcrumb_persistent_storage_manager_;
 
-  // Must be destroyed after |local_state_|.
+  // Must be destroyed after |local_state_|. BrowserStatePolicyConnector isn't a
+  // keyed service because the pref service, which isn't a keyed service, has a
+  // hard dependency on the policy infrastructure. In order to outlive the pref
+  // service, the policy connector must live outside the keyed services.
   std::unique_ptr<BrowserPolicyConnectorIOS> browser_policy_connector_;
 
   std::unique_ptr<PrefService> local_state_;

ios/chrome/browser/policy/browser_state_policy_connector.h

@@ -18,6 +18,10 @@ class SchemaRegistry;
 
 // BrowserStatePolicyConnector creates and manages the per-BrowserState policy
 // components and their integration with PrefService.
+// BrowserStatePolicyConnector isn't a keyed service because the pref service,
+// which isn't a keyed service, has a hard dependency on the policy
+// infrastructure. In order to outlive the pref service, the policy connector
+// must live outside the keyed services.
 class BrowserStatePolicyConnector {
  public:
   BrowserStatePolicyConnector();

ios/chrome/browser/policy/browser_state_policy_connector_mock.h

@@ -16,6 +16,10 @@ class SchemaRegistry;
 
 // BrowserStatePolicyConnector creates and manages the per-BrowserState policy
 // components and their integration with PrefService.
+// BrowserStatePolicyConnector isn't a keyed service because the pref service,
+// which isn't a keyed service, has a hard dependency on the policy
+// infrastructure. In order to outlive the pref service, the policy connector
+// must live outside the keyed services.
 class BrowserStatePolicyConnectorMock : public BrowserStatePolicyConnector {
  public:
   BrowserStatePolicyConnectorMock(

ios/chrome/test/testing_application_context.h

@@ -70,8 +70,13 @@ class TestingApplicationContext : public ApplicationContext {
   base::ThreadChecker thread_checker_;
   std::string application_locale_;
   PrefService* local_state_;
-  // Must be destroyed after |local_state_|.
+
+  // Must be destroyed after |local_state_|. BrowserStatePolicyConnector isn't a
+  // keyed service because the pref service, which isn't a keyed service, has a
+  // hard dependency on the policy infrastructure. In order to outlive the pref
+  // service, the policy connector must live outside the keyed services.
   std::unique_ptr<BrowserPolicyConnectorIOS> browser_policy_connector_;
+
   ios::ChromeBrowserStateManager* chrome_browser_state_manager_;
   std::unique_ptr<network_time::NetworkTimeTracker> network_time_tracker_;
   bool was_last_shutdown_clean_;