mash: Fix screen lock preload crash after login

chrome/browser/chromeos/login/lock/webui_screen_locker.cc

Index: chrome/browser/chromeos/login/lock/webui_screen_locker.cc
diff --git a/chrome/browser/chromeos/login/lock/webui_screen_locker.cc b/chrome/browser/chromeos/login/lock/webui_screen_locker.cc
index 659e0f45c4c65f9d38d843ead6cc2dc73022d588..f9ad40de117466886e5c58a29443caf172ffe805 100644
--- a/chrome/browser/chromeos/login/lock/webui_screen_locker.cc
+++ b/chrome/browser/chromeos/login/lock/webui_screen_locker.cc
@@ -21,6 +21,7 @@
 #include "chrome/browser/chromeos/login/ui/preloaded_web_view_factory.h"
 #include "chrome/browser/chromeos/login/ui/webui_login_display.h"
 #include "chrome/browser/chromeos/profiles/profile_helper.h"
+#include "chrome/browser/ui/ash/ash_util.h"
 #include "chrome/browser/ui/webui/chromeos/login/oobe_ui.h"
 #include "chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h"
 #include "chrome/common/chrome_features.h"
@@ -81,6 +82,12 @@ void WebUIScreenLocker::RequestPreload() {
 
 // static
 bool WebUIScreenLocker::ShouldPreloadLockScreen() {
+  // Bail for mash because IdleDetector/UserActivityDetector does not work
+  // properly there.
+  // TODO(xiyuan): Revisit after http://crbug.com/626899.
+  if (ash_util::IsRunningInMash())

[Daniel Erat, 2017/03/06 23:48:58]

the connection between this code and IdleDetector isn't obvious from looking at
this file. from the stack trace in crbug.com/698889, i'm guessing that
PreloadedWebView tries to defer loading the page until the user is idle or
something like that, right? would it be better to make PreloadedWebView just
load the page immediately when running under mash?

[xiyuan, 2017/03/06 23:54:50]

Your reading is correct. This is an optimization to make the lock screen show up
faster. Screen lock works without the preloading.

I don't want to preload it for mash because preloading makes it unnecessarily
slow and may make it unnecessarily complicated for mash work. Let's keep it
plain and simple for now. After we fix IdleDetector/UserActivityDetector for
chrome_on_mash, we can bring back the feature easily.

[Daniel Erat, 2017/03/07 01:27:16]

thanks for the explanation! would you mind also filing a mash bug to make sure
that we don't forget to wire this back up? :-) (or are you planning to retitle
crbug.com/698889 and use it?)

[xiyuan, 2017/03/07 17:00:07]

SG. Filed http://crbug.com/699144 to restore the reloading after 698889. 

+    return false;
+
   Profile* profile = ProfileHelper::Get()->GetProfileByUser(
       user_manager::UserManager::Get()->GetActiveUser());