replace size-alignment assert with round-down

Don't need to assert here, we just won't write into all of the external memory
if its length is not a multiple of what we need.

Bug: skia:
Change-Id: I277653f5dc3b3d7560ab71a10440275f55430833
Reviewed-on: https://skia-review.googlesource.com/118884
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed