initial preroll api

src/core/SkPictureShader.cpp

Index: src/core/SkPictureShader.cpp
diff --git a/src/core/SkPictureShader.cpp b/src/core/SkPictureShader.cpp
index 819a0d0ae71c0451cb2b206423e2e1c158fc00f4..36b55cae6666053357ac73dd0d6f0c85727476d8 100644
--- a/src/core/SkPictureShader.cpp
+++ b/src/core/SkPictureShader.cpp
@@ -221,6 +221,10 @@ SkShader::Context* SkPictureShader::onCreateContext(const ContextRec& rec, void*
     return PictureShaderContext::Create(storage, *this, rec, bitmapShader);
 }
 
+void SkPictureShader::onPreroll() const {
+    fPicture->preroll(NULL, NULL, NULL, NULL);
+}
+
 /////////////////////////////////////////////////////////////////////////////////////////
 
 SkShader::Context* SkPictureShader::PictureShaderContext::Create(void* storage,