Add image->bitmap

BUG=skia:

patch from issue 1212163012 at patchset 1 (http://crrev.com/1212163012#ps1)

Review URL: https://codereview.chromium.org/1208993017
diff --git a/src/image/SkImage_Base.h b/src/image/SkImage_Base.h
index 0793abb..b3661d9 100644
--- a/src/image/SkImage_Base.h
+++ b/src/image/SkImage_Base.h
@@ -61,6 +61,8 @@
                                 SkFilterQuality) const;
     virtual SkData* onRefEncoded() const { return NULL; }
 
+    virtual bool onAsLegacyBitmap(SkBitmap*, LegacyBitmapMode) const;
+
 private:
     const SkSurfaceProps fProps;