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

ash/shell/shell_delegate_impl.cc

Index: ash/shell/shell_delegate_impl.cc
diff --git a/ash/shell/shell_delegate_impl.cc b/ash/shell/shell_delegate_impl.cc
index ce07345dbeb595ddd3c4fd1548a5c85cf17ba39f..f4c735cc817c2fe3292e0ac354ab961d2678c608 100644
--- a/ash/shell/shell_delegate_impl.cc
+++ b/ash/shell/shell_delegate_impl.cc
@@ -266,6 +266,10 @@ GPUSupport* ShellDelegateImpl::CreateGPUSupport() {
   return new GPUSupportStub;
 }
 
+UserMetricsRecorderProxy* ShellDelegateImpl::CreateUserMetricsRecorderProxy() {
+  return nullptr;
+}
+
 base::string16 ShellDelegateImpl::GetProductName() const {
   return base::string16();
 }