Asynchronously retrieves a snapshot of this for region.
options specifies how the snapshot should be rendered.
When the operation is finished, callback will be called. You must call get_snapshot.end to get
the result of the operation.
| this |
a WebView |
| region |
the SnapshotRegion for this snapshot |
| options |
SnapshotOptions for the snapshot |
| cancellable |
a Cancellable |
| callback |
a TaskReadyCallback |
| user_data |
user data |