Make WindowReachedScreen support async readback

ui/snapshot/snapshot.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_SNAPSHOT_SNAPSHOT_H_
 #define UI_SNAPSHOT_SNAPSHOT_H_
 
 #include <vector>
 
 #include "base/callback_forward.h"
@@ skipping 10 matching lines @@
 class Rect;
 class Image;
 class Size;
 }
 
 namespace ui {
 
 // Grabs a snapshot of the window/view. No security checks are done. This is
 // intended to be used for debugging purposes where no BrowserProcess instance
 // is available (ie. tests). This function is synchronous, so it should NOT be
-// used in a result of user action. Use asynchronous GrabWindowSnapshotAsync()
-// instead on supported platforms.
+// used in a result of user action. Support for async vs synchronous
+// GrabWindowSnapshot differs by platform.  To be most general, use the
+// synchronous function first and if it returns false call the async one.
 SNAPSHOT_EXPORT bool GrabWindowSnapshot(
     gfx::NativeWindow window,
     std::vector<unsigned char>* png_representation,
     const gfx::Rect& snapshot_bounds);
 
 SNAPSHOT_EXPORT bool GrabViewSnapshot(
     gfx::NativeView view,
     std::vector<unsigned char>* png_representation,
     const gfx::Rect& snapshot_bounds);
 
 typedef base::Callback<void(const gfx::Image& snapshot)>
     GrabWindowSnapshotAsyncCallback;
 // GrabWindowSnapshotAndScaleAsync() copies snapshot of |source_rect| from
 // window and scales it to |target_size| asynchronously.
 SNAPSHOT_EXPORT void GrabWindowSnapshotAndScaleAsync(
     gfx::NativeWindow window,
     const gfx::Rect& source_rect,
     const gfx::Size& target_size,
     scoped_refptr<base::TaskRunner> background_task_runner,
     const GrabWindowSnapshotAsyncCallback& callback);
 
 typedef base::Callback<void(scoped_refptr<base::RefCountedBytes> png_data)>
     GrabWindowSnapshotAsyncPNGCallback;
 SNAPSHOT_EXPORT void GrabWindowSnapshotAsync(
     gfx::NativeWindow window,
     const gfx::Rect& source_rect,
     scoped_refptr<base::TaskRunner> background_task_runner,
     const GrabWindowSnapshotAsyncPNGCallback& callback);
+SNAPSHOT_EXPORT void GrabViewSnapshotAsync(
+    gfx::NativeView view,
+    const gfx::Rect& source_rect,
+    scoped_refptr<base::TaskRunner> background_task_runner,
+    const GrabWindowSnapshotAsyncPNGCallback& callback);
 
 }  // namespace ui
 
 #endif  // UI_SNAPSHOT_SNAPSHOT_H_