add preroll to image, for chrome's warmup pass

src/image/SkImage_Raster.cpp

Index: src/image/SkImage_Raster.cpp
diff --git a/src/image/SkImage_Raster.cpp b/src/image/SkImage_Raster.cpp
index 015f98a646b37368b881ccde46a833de0311f91d..af8f79010f134047b0ebf25678f9a22364dcb0d3 100644
--- a/src/image/SkImage_Raster.cpp
+++ b/src/image/SkImage_Raster.cpp
@@ -64,6 +64,7 @@ public:
 
     SkSurface* onNewSurface(const SkImageInfo&, const SkSurfaceProps&) const override;
     bool onReadPixels(const SkImageInfo&, void*, size_t, int srcX, int srcY) const override;
+    void onPreroll(GrContext*) const override;
     const void* onPeekPixels(SkImageInfo*, size_t* /*rowBytes*/) const override;
     SkData* onRefEncoded() const override;
     bool getROPixels(SkBitmap*) const override;
@@ -163,6 +164,11 @@ const void* SkImage_Raster::onPeekPixels(SkImageInfo* infoPtr, size_t* rowBytesP
     return fBitmap.getPixels();
 }
 
+void SkImage_Raster::onPreroll(GrContext* ctx) const {
+    fBitmap.lockPixels();
+    fBitmap.unlockPixels();
+}
+
 SkData* SkImage_Raster::onRefEncoded() const {
     SkPixelRef* pr = fBitmap.pixelRef();
     const SkImageInfo prInfo = pr->info();