Make WindowReachedScreen support async readback

ui/snapshot/snapshot.h

Index: ui/snapshot/snapshot.h
diff --git a/ui/snapshot/snapshot.h b/ui/snapshot/snapshot.h
index 53a698323a396f920471f6040faaa5f462aa978a..e547e4748ddaf77b8a4e150f420556bbe3726e0c 100644
--- a/ui/snapshot/snapshot.h
+++ b/ui/snapshot/snapshot.h
@@ -28,8 +28,9 @@ 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,
@@ -58,6 +59,11 @@ SNAPSHOT_EXPORT void GrabWindowSnapshotAsync(
     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