Fix login visual hitch on chromebook

chrome/browser/chromeos/extensions/wallpaper_private_api.cc

Index: chrome/browser/chromeos/extensions/wallpaper_private_api.cc
diff --git a/chrome/browser/chromeos/extensions/wallpaper_private_api.cc b/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
index 10ae1a1cd20f615afdc4e9441775ad7c3f1eea83..f20fc745bb7ffd136271dc4a1f9854c65e65f677 100644
--- a/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
+++ b/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
@@ -230,9 +230,12 @@ class WallpaperFunctionBase::WallpaperDecoder : public ImageDecoder::Delegate {
   }
 
   void Start(const std::string& image_data) {
+    DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
     image_decoder_ = new ImageDecoder(this, image_data,
                                       ImageDecoder::ROBUST_JPEG_CODEC);
-    image_decoder_->Start();
+    scoped_refptr<base::MessageLoopProxy> task_runner =
+        BrowserThread::GetMessageLoopProxyForThread(BrowserThread::UI);
+    image_decoder_->Start(task_runner);
   }
 
   void Cancel() {