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

content/public/browser/user_metrics.h

Index: content/public/browser/user_metrics.h
diff --git a/content/public/browser/user_metrics.h b/content/public/browser/user_metrics.h
index 118d8a93ae3a81cb8d5cc2924cd8ff2553e0c20a..950643677b819ab62e29fe183a3e961816db364a 100644
--- a/content/public/browser/user_metrics.h
+++ b/content/public/browser/user_metrics.h
@@ -13,6 +13,8 @@
 
 namespace content {
 
+// TODO(beaudoin): Get rid of these methods now that the base:: version does
+// thread hopping. Tracked in crbug.com/601483.
 // Wrappers around functions defined in base/metrics/user_metrics.h, refer to
 // that header for full documentation. These wrappers can be called from any
 // thread (they will post back to the UI thread to do the recording).