make SkColorShader private, use public factory

BUG=skia:

Need https://codereview.chromium.org/785823002/ to land first

Review URL: https://codereview.chromium.org/788533002
diff --git a/gyp/core.gypi b/gyp/core.gypi
index 2c6d7b1..63c9ca0 100644
--- a/gyp/core.gypi
+++ b/gyp/core.gypi
@@ -53,6 +53,7 @@
         '<(skia_src_path)/core/SkClipStack.cpp',
         '<(skia_src_path)/core/SkColor.cpp',
         '<(skia_src_path)/core/SkColorFilter.cpp',
+        '<(skia_src_path)/core/SkColorShader.h',
         '<(skia_src_path)/core/SkColorTable.cpp',
         '<(skia_src_path)/core/SkComposeShader.cpp',
         '<(skia_src_path)/core/SkConfig8888.cpp',
@@ -243,7 +244,6 @@
         '<(skia_include_path)/core/SkColor.h',
         '<(skia_include_path)/core/SkColorFilter.h',
         '<(skia_include_path)/core/SkColorPriv.h',
-        '<(skia_include_path)/core/SkColorShader.h',
         '<(skia_include_path)/core/SkComposeShader.h',
         '<(skia_include_path)/core/SkData.h',
         '<(skia_include_path)/core/SkDeque.h',