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>
diff --git a/src/core/SkColorSpaceXformer.cpp b/src/core/SkColorSpaceXformer.cpp
index 5329294..1e56d4d 100644
--- a/src/core/SkColorSpaceXformer.cpp
+++ b/src/core/SkColorSpaceXformer.cpp
@@ -6,6 +6,7 @@
  */
 
 #include "SkColorFilter.h"
+#include "SkColorSpacePriv.h"
 #include "SkColorSpaceXformer.h"
 #include "SkColorSpaceXformPriv.h"
 #include "SkDrawLooper.h"
@@ -26,7 +27,7 @@
 
 std::unique_ptr<SkColorSpaceXformer> SkColorSpaceXformer::Make(sk_sp<SkColorSpace> dst) {
     std::unique_ptr<SkColorSpaceXform> fromSRGB = SkMakeColorSpaceXform(
-            SkColorSpace::MakeSRGB().get(), dst.get());
+            sk_srgb_singleton(), dst.get());
 
     return fromSRGB
         ? std::unique_ptr<SkColorSpaceXformer>(new SkColorSpaceXformer(std::move(dst),