[Sync] SyncEngine refactor part 1: interfaces.

components/sync/engine/sync_engine_host.h

Index: components/sync/engine/sync_engine_host.h
diff --git a/components/sync/driver/sync_frontend.h b/components/sync/engine/sync_engine_host.h
similarity index 84%
rename from components/sync/driver/sync_frontend.h
rename to components/sync/engine/sync_engine_host.h
index c68b96a8801b606e9a8289bec10c9a2cc34fdfe4..55b670b5717333e5c51c707ba363d357aae94e60 100644
--- a/components/sync/driver/sync_frontend.h
+++ b/components/sync/engine/sync_engine_host.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef COMPONENTS_SYNC_DRIVER_SYNC_FRONTEND_H_
-#define COMPONENTS_SYNC_DRIVER_SYNC_FRONTEND_H_
+#ifndef COMPONENTS_SYNC_ENGINE_SYNC_ENGINE_HOST_H_
+#define COMPONENTS_SYNC_ENGINE_SYNC_ENGINE_HOST_H_
 
 #include <string>
 
@@ -26,15 +26,13 @@ struct CommitCounters;
 struct StatusCounters;
 struct UpdateCounters;
 
-// SyncFrontend is the interface used by SyncBackendHost to communicate with
-// the entity that created it and, presumably, is interested in sync-related
-// activity.
-// NOTE: All methods will be invoked by a SyncBackendHost on the same thread
-// used to create that SyncBackendHost.
-class SyncFrontend {
+// SyncEngineHost is the interface used by SyncEngine to communicate with the
+// entity that created it. It's essentially an observer interface except the
+// SyncEngine always has exactly one.
+class SyncEngineHost {
  public:
-  SyncFrontend();
-  virtual ~SyncFrontend();
+  SyncEngineHost();
+  virtual ~SyncEngineHost();
 
   // The backend has completed initialization and it is now ready to
   // accept and process changes.  If success is false, initialization
@@ -52,11 +50,10 @@ class SyncFrontend {
   // The backend queried the server recently and received some updates.
   virtual void OnSyncCycleCompleted() = 0;
 
-  // Informs the frontned of some network event.  These notifications are
-  // disabled by default and must be enabled through an explicit request to the
-  // SyncBackendHost.
+  // Informs the host of some network event. These notifications are disabled by
+  // default and must be enabled through an explicit request to the SyncEngine.
   //
-  // It's disabld by default to avoid copying data across threads when no one
+  // It's disabled by default to avoid copying data across threads when no one
   // is listening for it.
   virtual void OnProtocolEvent(const ProtocolEvent& event) = 0;
 
@@ -91,7 +88,7 @@ class SyncFrontend {
   // called when the first sensitive data type is setup by the user and anytime
   // the passphrase is changed by another synced client. |reason| denotes why
   // the passphrase was required. |pending_keys| is a copy of the
-  // cryptographer's pending keys to be passed on to the frontend in order to
+  // cryptographer's pending keys to be passed on to the host in order to
   // be cached.
   virtual void OnPassphraseRequired(
       PassphraseRequiredReason reason,
@@ -124,7 +121,7 @@ class SyncFrontend {
   // Called to perform migration of |types|.
   virtual void OnMigrationNeededForTypes(ModelTypeSet types) = 0;
 
-  // Inform the Frontend that new datatypes are available for registration.
+  // Called when new datatypes are available for registration.
   virtual void OnExperimentsChanged(const Experiments& experiments) = 0;
 
   // Called when the sync cycle returns there is an user actionable error.
@@ -141,4 +138,4 @@ class SyncFrontend {
 
 }  // namespace syncer
 
-#endif  // COMPONENTS_SYNC_DRIVER_SYNC_FRONTEND_H_
+#endif  // COMPONENTS_SYNC_ENGINE_SYNC_ENGINE_HOST_H_