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

ash/test/test_shell_delegate.h

Index: ash/test/test_shell_delegate.h
diff --git a/ash/test/test_shell_delegate.h b/ash/test/test_shell_delegate.h
index 9b273fc9a48134bae5f9f2e6d7420b1e8ee5fdba..1668de55097efe44b65f0b500412c950c6f3ff5c 100644
--- a/ash/test/test_shell_delegate.h
+++ b/ash/test/test_shell_delegate.h
@@ -59,6 +59,7 @@ class TestShellDelegate : public ShellDelegate {
                                    ash::ShelfItemDelegate* item_delegate,
                                    ash::ShelfItem* item) override;
   GPUSupport* CreateGPUSupport() override;
+  UserMetricsRecorderProxy* CreateUserMetricsRecorderProxy() override;
   base::string16 GetProductName() const override;
 
   int num_exit_requests() const { return num_exit_requests_; }