initial preroll api

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index f8de276c7bff4836a36f5298e1e551c6ef86be41..1ae531e2e5d95294af98735a229a0e31fdfc9dc0 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -478,6 +478,8 @@ public:
      */
     virtual SkShader* refAsALocalMatrixShader(SkMatrix* localMatrix) const;
 
+    void preroll() const { this->onPreroll(); }
+
     SK_TO_STRING_VIRT()
     SK_DEFINE_FLATTENABLE_TYPE(SkShader)
 
@@ -492,6 +494,7 @@ protected:
      */
     virtual Context* onCreateContext(const ContextRec&, void* storage) const;
 
+    virtual void onPreroll() const {}
     virtual bool onAsLuminanceColor(SkColor*) const {
         return false;
     }