Added asNewCustomStage to SkShader and implemented it for all the gradient shaders.

Not actually hooked up yet, but it should be ready to replace asABitmap for a future CL.
Review URL: https://codereview.appspot.com/6351113

git-svn-id: http://skia.googlecode.com/svn/trunk@4702 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/core/SkShader.cpp b/src/core/SkShader.cpp
index cd40b7f..a0ae49d 100644
--- a/src/core/SkShader.cpp
+++ b/src/core/SkShader.cpp
@@ -200,6 +200,11 @@
     return kNone_GradientType;
 }
 
+GrCustomStage* SkShader::asNewCustomStage(GrContext* context, 
+                                          GrSamplerState* sampler) const {
+    return NULL;
+}
+
 SkShader* SkShader::CreateBitmapShader(const SkBitmap& src,
                                        TileMode tmx, TileMode tmy) {
     return SkShader::CreateBitmapShader(src, tmx, tmy, NULL, 0);