Reopen a Guest browser and not the User Manager if another Guest browser is already open.

chrome/browser/ui/startup/startup_browser_creator.cc

Index: chrome/browser/ui/startup/startup_browser_creator.cc
diff --git a/chrome/browser/ui/startup/startup_browser_creator.cc b/chrome/browser/ui/startup/startup_browser_creator.cc
index b972539badf3da9b079db1ee3a62da9e72eb4b59..237973d8d0507b7eb18e214111a0a44e3ca113a6 100644
--- a/chrome/browser/ui/startup/startup_browser_creator.cc
+++ b/chrome/browser/ui/startup/startup_browser_creator.cc
@@ -633,15 +633,25 @@ bool StartupBrowserCreator::ProcessCmdLineImpl(
       bool signin_required = profile_index != std::string::npos &&
           profile_info.ProfileIsSigninRequiredAtIndex(profile_index);
 
-      // Guest or locked profiles cannot be re-opened on startup.
-      if (signin_required || last_used_profile->IsGuestSession()) {
+      // Guest or locked profiles cannot be re-opened on startup. The only
+      // exception is if there's already a Guest window open in a separate
+      // process (for example, launching a new browser after clicking on a
+      // downloaded file in Guest mode).
+      size_t guest_browsers = last_used_profile->IsGuestSession() ?
+          chrome::GetTotalBrowserCountForProfile(
+              last_used_profile->GetOffTheRecordProfile()) : -1;

[sky, 2014/10/30 21:17:51]

I suspect the -1 to a size_t is going to cause warnings. You don't really the
size though right, just if it's empty. Can you convert to a boolean here?

[noms (inactive), 2014/10/31 19:52:10]

Done. Also fixed the logic a bit (there needs to be the extra check for the
guest profile or else it gets triggered for regular profiles with no incognito
browsers...)

+      if (signin_required || (guest_browsers == 0)) {
         UserManager::Show(base::FilePath(),
                           profiles::USER_MANAGER_NO_TUTORIAL,
                           profiles::USER_MANAGER_SELECT_PROFILE_NO_ACTION);
         return true;
       }
     }
-    if (!browser_creator->LaunchBrowser(command_line, last_used_profile,
+
+    Profile* profile_to_open = last_used_profile->IsGuestSession() ?
+        last_used_profile->GetOffTheRecordProfile() : last_used_profile;
+
+    if (!browser_creator->LaunchBrowser(command_line, profile_to_open,
                                         cur_dir, is_process_startup,
                                         is_first_run, return_code)) {
       return false;