Const-qualify SVGListPropertyHelper::Clone()

Should not mutate the object/list.

Change-Id: I3efe2e7726b2671b32ac7d9a0bc44b48621bcd69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2480823Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#818055}

third_party/blink/renderer/core/svg/properties/svg_list_property_helper.h

@@ -100,9 +100,9 @@ class SVGListPropertyHelper : public SVGListPropertyBase {
     SVGListPropertyBase::Replace(index, new_item);
   }
 
-  virtual Derived* Clone() {
+  virtual Derived* Clone() const {
     auto* svg_list = MakeGarbageCollected<Derived>();
-    svg_list->DeepCopy(static_cast<Derived*>(this));
+    svg_list->DeepCopy(To<Derived>(this));
     return svg_list;
   }
 

third_party/blink/renderer/core/svg/svg_length_list.cc

@@ -30,7 +30,7 @@ SVGLengthList::SVGLengthList(SVGLengthMode mode) : mode_(mode) {}
 
 SVGLengthList::~SVGLengthList() = default;
 
-SVGLengthList* SVGLengthList::Clone() {
+SVGLengthList* SVGLengthList::Clone() const {
   auto* ret = MakeGarbageCollected<SVGLengthList>(mode_);
   ret->DeepCopy(this);
   return ret;

third_party/blink/renderer/core/svg/svg_length_list.h

@@ -52,7 +52,7 @@ class SVGLengthList final
 
   // SVGPropertyBase:
   SVGPropertyBase* CloneForAnimation(const String&) const override;
-  SVGLengthList* Clone() override;
+  SVGLengthList* Clone() const override;
   SVGLengthMode UnitMode() const { return mode_; }
 
   void Add(const SVGPropertyBase*, const SVGElement*) override;