Add fallback behavior if the last used profile cannot initialize

chrome/browser/ui/startup/startup_browser_creator.h

Index: chrome/browser/ui/startup/startup_browser_creator.h
diff --git a/chrome/browser/ui/startup/startup_browser_creator.h b/chrome/browser/ui/startup/startup_browser_creator.h
index f2e3c0a5fd757609b5df07e3960161ba5350888d..6e00e0cf0dec6eda7c40847593039f34de0a1e8e 100644
--- a/chrome/browser/ui/startup/startup_browser_creator.h
+++ b/chrome/browser/ui/startup/startup_browser_creator.h
@@ -179,4 +179,18 @@ bool HasPendingUncleanExit(Profile* profile);
 base::FilePath GetStartupProfilePath(const base::FilePath& user_data_dir,
                                      const base::CommandLine& command_line);
 
+#if !defined(OS_CHROMEOS) && !defined(OS_ANDROID)
+// Returns the profile that should be loaded on process startup. This is either
+// the profile return by GetStartupProfilePath, or the guest profile (user
+// manager) if the above profile is signed out. Returns NULL if this profile

[Peter Kasting, 2016/06/16 06:37:49]

Nit: null (2 places)

What does "user manager" mean here?  Even after reading the code in the .cc file
I'm not sure how to best describe this.  Be more detailed.

[WC Leung, 2016/06/19 17:56:19]

Done.
The user manager (or UserManager) is an interface can usually be opened by
(1) left click you name (or icon) in right side of the title bar.
(2) select "Switch Person".
The underlying url of the user manager is chrome://user-manager, which is
supposed to be opened in the system profile.

Not sure whether I should add a elaborate explanation in the source code,
because it is going to be long.

Note: There are a few different interfaces for switching profiles. The user
manager is one of them. Another is called profile choose view, which is opened
when you right click your name in the title bar. And yet another can be found in
chrome://settings.

[Peter Kasting, 2016/06/20 02:25:50]

Sorry, I wasn't sufficiently clear.  I know what the user manager is.  What I'm
confused by is the construction of a sentence which places "(user manager)"
after "guest profile" as if the user manager is another name for the guest
profile.  It's like reading a sentence like "I went to the store and bought two
cucumbers (football team)."  "football team" is mystifying there.

So I meant, clarify how the user manager relates to what's returned.

This is related to the issue where it seems strange that we return the guest
profile if we intend to show the user manager to begin with.

[WC Leung, 2016/06/20 15:44:24]

Acknowledged.
Looks like we should return a structure with a enum and optionally a profile, in
which the enum class looks like OpenAtStartup {PROFILE, USER_MANAGER}. Returning
a profile to request the user manager is confusing, and it blocks the
possibility to open the real guest profile. (If the user manager fails to show,
I believe that we should fallback to the real guest profile.)

It's likely that we should start a new issue for this. (But well, this is
already the third issue that we have come up with for one single issue.)

Mike: 
Do you know what was happening with this code in the past? Why do we return the
guest profile to denote the user manager? Was there any special reason in the
design?

[WC Leung, 2016/06/24 16:17:50]

Done.

+// cannot be opened.
+Profile* GetStartupProfile(const base::FilePath& user_data_dir,
+                           const base::CommandLine& command_line);
+
+// Returns the a fallback profile that should be loaded on process startup.
+// Possible to return NULL, which means no profile (including guest profile) can
+// be opened.
+Profile* GetFallbackStartupProfile();
+#endif  // !defined(OS_CHROMEOS) && !defined(OS_ANDROID)
+
 #endif  // CHROME_BROWSER_UI_STARTUP_STARTUP_BROWSER_CREATOR_H_