Force guest profiles to return default startup prefs.

chrome/browser/prefs/session_startup_pref.cc

Index: chrome/browser/prefs/session_startup_pref.cc
diff --git a/chrome/browser/prefs/session_startup_pref.cc b/chrome/browser/prefs/session_startup_pref.cc
index 9126203deeba9a3b95501a654cc9d16baccc036e..d8beaeda6a5d01fb31201891558e14b0ea9dbd84 100644
--- a/chrome/browser/prefs/session_startup_pref.cc
+++ b/chrome/browser/prefs/session_startup_pref.cc
@@ -99,7 +99,18 @@ void SessionStartupPref::SetStartupPref(PrefService* prefs,
 // static
 SessionStartupPref SessionStartupPref::GetStartupPref(Profile* profile) {
   DCHECK(profile);
-  return GetStartupPref(profile->GetPrefs());
+
+  PrefService* prefs = profile->GetPrefs();
+
+  // If a guest profile, force a return of the DEFAULT startup prefs. See

[sky, 2017/06/20 16:45:27]

When writing comments strive for comments that clarify 'why' the code is the way
it is. For example, here I think you want:

// Guest sessions should not store any state and such never trigger
// a restore during startup.

[shrike, 2017/06/20 18:01:17]

Acknowledged.

+  // https://crbug.com/723414 .
+  if (profile->IsGuestSession()) {
+    DCHECK(prefs);
+    prefs->SetInteger(prefs::kRestoreOnStartup,

[sky, 2017/06/20 16:45:27]

Is it really necessary to modify the prefs here? Instead could you return a
SessionStartPref with the type explicitly set to default, e.g.:

return SessionStartPref(SessionStartPref::DEFAULT);
?

[shrike, 2017/06/20 18:01:17]

I was concerned about not changing the original code path too much, but I see
that SessionStartupPref::GetStartupPref() should not do much of anything with a
DEFAULT profile, so should be safe to bypass it.

+                      TypeToPrefValue(SessionStartupPref::DEFAULT));
+  }
+
+  return GetStartupPref(prefs);
 }
 
 // static