add explicit accessor for sRGB singleton colorspaces

SkColorSpace::MakeSRGB().get() is scary, and causes more ref/unref
pairs than strictly necessary for these singletons.

This time the implementation is still in SkColorSpace.cpp,
so these should really work as singletons.

Change-Id: I40f2942c8dcde3040663a04c4f5330aca90868ae
Reviewed-on: https://skia-review.googlesource.com/143305
Auto-Submit: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
15 files changed