make compute helper for blurs private

Precursor for moving blurmaskfilter into core, since it is referenced
by core code for drawShadow.

Bug: skia:
Change-Id: I900c6e10523115c75f45d2c410eb6a5ca56a6e4d
Reviewed-on: https://skia-review.googlesource.com/112301
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Mike Reed <reed@google.com>
7 files changed