[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..a8bcefece83efe049e2e1bba90c34c8479923ad4 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 +150,11 @@ 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|,
+  // which should be wrapped with syncer::BindToCurrentThread already.
+  // Used to display entity counts in chrome://sync-internals.
+  virtual void GetStatusCounters(const StatusCountersCallback& callback) = 0;
+
  protected:
   DataTypeController(ModelType type, const base::Closure& dump_stack);