initial preroll api

src/core/SkBitmapProcShader.cpp

Index: src/core/SkBitmapProcShader.cpp
diff --git a/src/core/SkBitmapProcShader.cpp b/src/core/SkBitmapProcShader.cpp
index 456e0d492087d29d84070ee1590b9a5da352aedc..e6ed1136c0e8066105c94a276aab3a680e1bc3af 100644
--- a/src/core/SkBitmapProcShader.cpp
+++ b/src/core/SkBitmapProcShader.cpp
@@ -112,6 +112,12 @@ size_t SkBitmapProcShader::contextSize() const {
     return sizeof(BitmapProcShaderContext) + sizeof(SkBitmapProcState);
 }
 
+void SkBitmapProcShader::onPreroll() const {
+    SkBitmap bm(fRawBitmap);
+    bm.lockPixels();
+    bm.unlockPixels();
+}
+
 SkBitmapProcShader::BitmapProcShaderContext::BitmapProcShaderContext(
         const SkBitmapProcShader& shader, const ContextRec& rec, SkBitmapProcState* state)
     : INHERITED(shader, rec)