Move the thread hop for UMA user actions from content:: to base::.

components/metrics/metrics_service_client.h

Index: components/metrics/metrics_service_client.h
diff --git a/components/metrics/metrics_service_client.h b/components/metrics/metrics_service_client.h
index 43c5d136ca8e00cc1f42e48b1d1c42e48362d575..3248f968614813c7fe6c86af29fcde543f9bbe8b 100644
--- a/components/metrics/metrics_service_client.h
+++ b/components/metrics/metrics_service_client.h
@@ -10,6 +10,7 @@
 
 #include "base/callback_forward.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/single_thread_task_runner.h"
 #include "base/strings/string16.h"
 #include "base/time/time.h"
 #include "components/metrics/proto/system_profile.pb.h"
@@ -99,6 +100,10 @@ class MetricsServiceClient {
   // Returns the standard interval between upload attempts.
   virtual base::TimeDelta GetStandardUploadInterval() = 0;
 
+  // Returns the task runner for the main thread.
+  virtual scoped_refptr<base::SingleThreadTaskRunner>
+  GetMainThreadTaskRunner() = 0;
+
   // Returns the name of a key under HKEY_CURRENT_USER that can be used to store
   // backups of metrics data. Unused except on Windows.
   virtual base::string16 GetRegistryBackupKey();