Fix ScreenLockerTest. Check whether bitmap is valid.

See also http://crbug.com/96032

BUG=124339
TBR=antrim@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10834169

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150018 0039d316-1c4b-4281-b951-d872f2087c98

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)),