Added the Ash.Tab.TimeBetweenSwitchToExistingTabUserActions histogram to track time between tab swit

ash/shell_delegate.h

Index: ash/shell_delegate.h
diff --git a/ash/shell_delegate.h b/ash/shell_delegate.h
index 64f81393fb0c76ca36b62b82e04b03a76c868833..e07dd2ccd362f2ed89f2cd144c3946c035070214 100644
--- a/ash/shell_delegate.h
+++ b/ash/shell_delegate.h
@@ -47,6 +47,7 @@ class ShelfDelegate;
 class ShelfItemDelegate;
 class ShelfModel;
 class SystemTrayDelegate;
+class UserMetricsRecorderProxy;
 class UserWallpaperDelegate;
 struct ShelfItem;
 
@@ -153,6 +154,9 @@ class ASH_EXPORT ShellDelegate {
   // Creates a GPU support object. Shell takes ownership of the object.
   virtual GPUSupport* CreateGPUSupport() = 0;
 
+  // TODO(bruthig):
+  virtual UserMetricsRecorderProxy* CreateUserMetricsRecorderProxy() = 0;
+
   // Get the product name.
   virtual base::string16 GetProductName() const = 0;
 };