Move SkGammaColorFilter to tools, limit to sRGB

Similarly, limit GrGammaEffect to sRGB (and rename it).

BUG=skia:

Change-Id: I88feef11ab7040bca2fa4c2eed71923ded87a0d0
Reviewed-on: https://skia-review.googlesource.com/7375
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 3013270..e7e629d 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -315,8 +315,6 @@
   "$_src/gpu/effects/GrDistanceFieldGeoProc.h",
   "$_src/gpu/effects/GrDitherEffect.cpp",
   "$_src/gpu/effects/GrDitherEffect.h",
-  "$_src/gpu/effects/GrGammaEffect.cpp",
-  "$_src/gpu/effects/GrGammaEffect.h",
   "$_src/gpu/effects/GrGaussianConvolutionFragmentProcessor.cpp",
   "$_src/gpu/effects/GrGaussianConvolutionFragmentProcessor.h",
   "$_src/gpu/effects/GrMatrixConvolutionEffect.cpp",
@@ -334,6 +332,8 @@
   "$_src/gpu/effects/GrSimpleTextureEffect.h",
   "$_src/gpu/effects/GrSingleTextureEffect.cpp",
   "$_src/gpu/effects/GrSingleTextureEffect.h",
+  "$_src/gpu/effects/GrSRGBEffect.cpp",
+  "$_src/gpu/effects/GrSRGBEffect.h",
   "$_src/gpu/effects/GrTextureDomain.cpp",
   "$_src/gpu/effects/GrTextureDomain.h",
   "$_src/gpu/effects/GrTextureStripAtlas.cpp",