BlendFormula to validate in constructor.

A now removed comment claimed that validation could not be done in the
constructor in C++11, but it turns out that it can be done. Move the
validation to construction time and remove all validation done at other
times.

Change-Id: I267d3063fac3dc702efae71841cbd7424b19af31
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278597
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed