Fix ScreenLockerTest. Check whether bitmap is valid.

chrome/browser/ui/webui/options2/chromeos/wallpaper_source.cc

Index: chrome/browser/ui/webui/options2/chromeos/wallpaper_source.cc
diff --git a/chrome/browser/ui/webui/options2/chromeos/wallpaper_source.cc b/chrome/browser/ui/webui/options2/chromeos/wallpaper_source.cc
index 70e1d09d04902aeb1abf6bf254e726514ca47d77..b1370e608a96b22b613477b1f736d557f2666a5f 100644
--- a/chrome/browser/ui/webui/options2/chromeos/wallpaper_source.cc
+++ b/chrome/browser/ui/webui/options2/chromeos/wallpaper_source.cc
@@ -57,6 +57,10 @@ class WallpaperImageSource::WallpaperEncodingOperation
       return;
     TRACE_EVENT0("LOCK_SCREEN", "imageEncoding");
     SkAutoLockPixels lock_input(image_);
+
+    if (!image_.readyToDraw())
+      return;
+
     // Avoid compression to make things faster.
     gfx::PNGCodec::EncodeWithCompressionLevel(
         reinterpret_cast<unsigned char*>(image_.getAddr32(0, 0)),