[USS] Show USS counters in about:sync page

components/sync/driver/data_type_controller.h

Index: components/sync/driver/data_type_controller.h
diff --git a/components/sync/driver/data_type_controller.h b/components/sync/driver/data_type_controller.h
index d3ab89d9642ff232a0a913dadab98408dade2865..848a26d0091a702aec301902da913678e30d60cc 100644
--- a/components/sync/driver/data_type_controller.h
+++ b/components/sync/driver/data_type_controller.h
@@ -16,6 +16,7 @@
 #include "components/sync/api/data_type_error_handler.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/base/unrecoverable_error_handler.h"
+#include "components/sync/engine/cycle/status_counters.h"
 
 namespace syncer {
 
@@ -70,6 +71,9 @@ class DataTypeController : public base::SupportsWeakPtr<DataTypeController> {
                               std::unique_ptr<base::ListValue>)>
       AllNodesCallback;
 
+  typedef base::Callback<void(ModelType, const StatusCounters&)>
+      StatusCountersCallback;
+
   typedef std::map<ModelType, std::unique_ptr<DataTypeController>> TypeMap;
   typedef std::map<ModelType, DataTypeController::State> StateMap;
 
@@ -146,6 +150,17 @@ class DataTypeController : public base::SupportsWeakPtr<DataTypeController> {
   // Used for populating nodes in Sync Node Browser of chrome://sync-internals.
   virtual void GetAllNodes(const AllNodesCallback& callback) = 0;
 
+  // Collects StatusCounters for this datatype and passes them to |callback|.

[maxbogue, 2016/10/06 17:16:32]

"Collects StatusCounters for this datatype and passes them to |callback|, which
should be wrapped with syncer::BindToCurrentThread already. Used to display
entity counts in chrome://sync-internals."

[Gang Wu, 2016/10/07 03:59:43]

Done.

+  // |callback| should wrapped with syncer::BindToCurrentThread already. then we
+  // can enforcing the asynchronous call for |callback|. Also, enforcing posting
+  // |callback| back to UI thread even collecting StatusCounters on UI thread,
+  // can prevent that results of ProfileSyncService::GetTypeStatusMap are
+  // applied to UI before sync counters collected here. Otherwise results of
+  // ProfileSyncService::GetTypeStatusMap will overwrite counters with empty
+  // data.
+  // Used for display counters in chrome://sync-internals.
+  virtual void GetStatusCounters(const StatusCountersCallback& callback) = 0;
+
  protected:
   DataTypeController(ModelType type, const base::Closure& dump_stack);