Make ChromeOS screenshots use async readback path

chrome/browser/ui/window_snapshot/window_snapshot.h

Index: chrome/browser/ui/window_snapshot/window_snapshot.h
diff --git a/chrome/browser/ui/window_snapshot/window_snapshot.h b/chrome/browser/ui/window_snapshot/window_snapshot.h
index cb681db739eb1cb3940ae43f8349fb3b135cf2ff..463b1b0a62ad53131eaad7d20c95c8218aaf05de 100644
--- a/chrome/browser/ui/window_snapshot/window_snapshot.h
+++ b/chrome/browser/ui/window_snapshot/window_snapshot.h
@@ -8,6 +8,7 @@
 #include <vector>
 
 #include "ui/gfx/native_widget_types.h"
+#include "ui/snapshot/snapshot.h"
 
 class PrefRegistrySimple;
 
@@ -27,10 +28,15 @@ void RegisterScreenshotPrefs(PrefRegistrySimple* registry);
 // by browser code. If you need to take a screenshot for debugging purposes,
 // consider using ui::GrabWindowSnapshot.
 // Returns true if the operation is successful (ie. permitted).
-bool GrabWindowSnapshotForUser(
+bool GrabWindowSnapshotForUser(gfx::NativeWindow window,
+                               std::vector<unsigned char>* png_representation,
+                               const gfx::Rect& snapshot_bounds);
+
+void GrabWindowSnapshotForUserAsync(
     gfx::NativeWindow window,
-    std::vector<unsigned char>* png_representation,
-    const gfx::Rect& snapshot_bounds);
+    const gfx::Rect& snapshot_bounds,
+    scoped_refptr<base::TaskRunner> background_task_runner,
+    ui::GrabWindowSnapshotAsyncPNGCallback callback);
 
 }  // namespace chrome