Force guest profiles to return default startup prefs.

chrome/browser/prefs/session_startup_pref.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/prefs/session_startup_pref.h"
 
 #include <stddef.h>
 
 #include <string>
 
@@ skipping 81 matching lines @@
     for (size_t i = 0; i < pref.urls.size(); ++i) {
       url_pref_list->Set(static_cast<int>(i),
                          base::MakeUnique<base::Value>(pref.urls[i].spec()));
     }
   }
 }
 
 // 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
 SessionStartupPref SessionStartupPref::GetStartupPref(PrefService* prefs) {
   DCHECK(prefs);
 
   SessionStartupPref pref(
       PrefValueToType(prefs->GetInteger(prefs::kRestoreOnStartup)));
 
   // Always load the urls, even if the pref type isn't URLS. This way the
@@ skipping 40 matching lines @@
     default:                 return SessionStartupPref::DEFAULT;
   }
 }
 
 SessionStartupPref::SessionStartupPref(Type type) : type(type) {}
 
 SessionStartupPref::SessionStartupPref(const SessionStartupPref& other) =
     default;
 
 SessionStartupPref::~SessionStartupPref() {}