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

ash/test/test_shell_delegate.cc

Index: ash/test/test_shell_delegate.cc
diff --git a/ash/test/test_shell_delegate.cc b/ash/test/test_shell_delegate.cc
index ee6fd44a06c28c7d52be65d43248b76dd3d5a52e..40da77dd074321096798f8966ff11bdcca255b44 100644
--- a/ash/test/test_shell_delegate.cc
+++ b/ash/test/test_shell_delegate.cc
@@ -195,6 +195,10 @@ GPUSupport* TestShellDelegate::CreateGPUSupport() {
   return new GPUSupportStub;
 }
 
+UserMetricsRecorderProxy* TestShellDelegate::CreateUserMetricsRecorderProxy() {
+  return nullptr;
+}
+
 base::string16 TestShellDelegate::GetProductName() const {
   return base::string16();
 }