We can use the Impl casting helper methods to clean up dynamic type
casting.

Change-Id: I5706da74eedd9f3cdc5a728420074a91ad7c95cb
Reviewed-on: https://chromium-review.googlesource.com/263520Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>