use std::enable_if instead of assert()

Crazy?  Genius?

Change-Id: I6033ab5f1af1a6bee84c27025b988e1143d59293
Reviewed-on: https://skia-review.googlesource.com/134512
Commit-Queue: Mike Klein <mtklein@chromium.org>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Mike Klein <mtklein@chromium.org>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed