Make cc::DrawingDisplayItem hold a const SkPicture.

cc/blink/web_display_item_list_impl.cc

Index: cc/blink/web_display_item_list_impl.cc
diff --git a/cc/blink/web_display_item_list_impl.cc b/cc/blink/web_display_item_list_impl.cc
index c46e8a0b3d999041b4a38ee7d24c149cf4085d9b..c9d8de745625ed0b4147a2b18907717b6397debc 100644
--- a/cc/blink/web_display_item_list_impl.cc
+++ b/cc/blink/web_display_item_list_impl.cc
@@ -54,10 +54,9 @@ void WebDisplayItemListImpl::appendDrawingItem(
     const SkPicture* picture) {
   if (display_item_list_->RetainsIndividualDisplayItems()) {
     display_item_list_->CreateAndAppendItem<cc::DrawingDisplayItem>(
-        visual_rect, skia::SharePtr(const_cast<SkPicture*>(picture)));
+        visual_rect, skia::SharePtr(picture));
   } else {
-    cc::DrawingDisplayItem item(
-        skia::SharePtr(const_cast<SkPicture*>(picture)));
+    cc::DrawingDisplayItem item(skia::SharePtr(picture));
     display_item_list_->RasterIntoCanvas(item);
   }
 }