Make cc::DrawingDisplayItem hold a const SkPicture.

cc/playback/drawing_display_item.cc

Index: cc/playback/drawing_display_item.cc
diff --git a/cc/playback/drawing_display_item.cc b/cc/playback/drawing_display_item.cc
index 78f26fe850c4cadfeee7dd17cb63e8f527f7e0b0..36181aed1b395bb05fea0ada15150a9b97ec904e 100644
--- a/cc/playback/drawing_display_item.cc
+++ b/cc/playback/drawing_display_item.cc
@@ -25,7 +25,7 @@ namespace cc {
 
 DrawingDisplayItem::DrawingDisplayItem() {}
 
-DrawingDisplayItem::DrawingDisplayItem(skia::RefPtr<SkPicture> picture) {
+DrawingDisplayItem::DrawingDisplayItem(skia::RefPtr<const SkPicture> picture) {
   SetNew(std::move(picture));
 }
 
@@ -51,7 +51,7 @@ DrawingDisplayItem::DrawingDisplayItem(const DrawingDisplayItem& item) {
 DrawingDisplayItem::~DrawingDisplayItem() {
 }
 
-void DrawingDisplayItem::SetNew(skia::RefPtr<SkPicture> picture) {
+void DrawingDisplayItem::SetNew(skia::RefPtr<const SkPicture> picture) {
   picture_ = std::move(picture);
 }