Asynchronously load wallpapers when user pod is selected.

chrome/browser/resources/chromeos/login/user_pod_row.js

Index: chrome/browser/resources/chromeos/login/user_pod_row.js
diff --git a/chrome/browser/resources/chromeos/login/user_pod_row.js b/chrome/browser/resources/chromeos/login/user_pod_row.js
index 149b3175eb580fab6afd6c04f64a8ce12a7dd471..f2dfe93cb476cd60d82f0e5266b75e5a62a774c8 100644
--- a/chrome/browser/resources/chromeos/login/user_pod_row.js
+++ b/chrome/browser/resources/chromeos/login/user_pod_row.js
@@ -647,6 +647,7 @@ cr.define('login', function() {
         podToFocus.classList.add('focused');
         podToFocus.reset(true);  // Reset and give focus.
         this.scrollPodIntoView(podToFocus);
+        chrome.send('userSelected', [podToFocus.user.username]);

[Nikita (slow), 2012/04/25 16:47:48]

See http://crosbug.com/1124339 it mentions one specific issue when we quickly
move through users as sign in screen.

Imagine selecting 3rd user with keyboard. First two would get selected quickly
so we need some timeout after user pod gets focus.

1. User pod focused
2. Set timeout (issue mentions 800ms)
3. When timeout is fired, check that same user pod is still focused and call
chrome.send('userSelectedDelayed') if it is still focused.

[bshe, 2012/04/25 23:54:43]

Done. I cleared the previous timeout and setup a new timeout when focused pod
changes. It should do the same (prevent loading wallpapers in between).
Although, 800ms seems a little too long to me, which combined with the 1 second
wallpaper loading time, makes the wallpaper changing sluggish (around 2 secs to
show the wallpaper). 

With the new preemption strategy, We can probably start to load the wallpaper
immediately. It only shows the wallpaper loaded successfully and discard all in
between wallpapers. This way we can reduce the wallpaper changing response time.
What do you think?

[Nikita (slow), 2012/04/26 17:22:03]

I still think that we should have some timeout and maybe experiment and decrease
it. Issue is that most time consuming operation won't be cancelled.

wallpaper_ = ui::ResourceBundle::GetSharedInstance().GetImageNamed(
  GetWallpaperInfo(index_).id).ToSkBitmap();

[bshe, 2012/04/26 20:25:21]

Make sense. I will keep it this way and using 800 as a start. We can adjust it
later.

       }
     },