Make Swizzle::MaskString a public method.

Some variation of this method was implemented in several places, so
might as well expose one implementation and use it everywhere.

Change-Id: I1da1a1df25ed2efbb95a0e0dbcbf988a31653f5f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/705231
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
6 files changed