[Mac] Track when the User Manager is opened.

Windows does this, Mac doesn't. ¯\(°_o)/¯

BUG=NONE

Review URL: https://codereview.chromium.org/590133003

Cr-Commit-Position: refs/heads/master@{#295977}

chrome/browser/ui/cocoa/profiles/user_manager_mac.mm

@@ -179,6 +179,7 @@ UserManagerMac::~UserManagerMac() {
 // static
 void UserManagerMac::Show(const base::FilePath& profile_path_to_focus,
                           profiles::UserManagerTutorialMode tutorial_mode) {
+  ProfileMetrics::LogProfileSwitchUser(ProfileMetrics::OPEN_USER_MANAGER);
   if (instance_) {
     // If there's a user manager window open already, just activate it.
     [instance_->window_controller_ show];