When casting a const value, ensure the cast is const as well.

BUG=webm:1509

Change-Id: I5b597fdcc148d111e9824bc7cf918fc5f24e970f
Reviewed-on: https://chromium-review.googlesource.com/996553Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>