Reland "use std::enable_if instead of assert()"

This is a reland of c6530d1e5ecce640e0255d66a01f9db73add7df9

Original change's description:
> 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>

Change-Id: I50849c89a37255396706ce659e20f00c0b0f08c6
Reviewed-on: https://skia-review.googlesource.com/134860
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed