[css-layout-api] Change TraceWrapperV8Reference to ScopedPersistent.

The ownership chain for the instance_ variable looks like:
LayoutCustom -> CSSLayoutDefinition::Instance -> v8::Object (instance_).

As LayoutCustom isn't a ScriptWrappable (lifetime is manually controlled)
using the TraceWrapperV8Reference was the wrong thing to do there.

Changed to ScopedPersistent.

Bug: 726125
Change-Id: I452a44e6914d042ca12f3bce3f8bce4a068bba5d
Reviewed-on: https://chromium-review.googlesource.com/986986Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547081}

third_party/WebKit/Source/core/layout/custom/CSSLayoutDefinition.cpp

@@ -274,11 +274,6 @@ void CSSLayoutDefinition::Instance::Trace(blink::Visitor* visitor) {
   visitor->Trace(definition_);
 }
 
-void CSSLayoutDefinition::Instance::TraceWrappers(
-    const ScriptWrappableVisitor* visitor) const {
-  visitor->TraceWrappers(instance_.Cast<v8::Value>());
-}
-
 void CSSLayoutDefinition::TraceWrappers(
     const ScriptWrappableVisitor* visitor) const {
   visitor->TraceWrappers(constructor_.Cast<v8::Value>());

third_party/WebKit/Source/core/layout/custom/CSSLayoutDefinition.h

@@ -7,6 +7,7 @@
 
 #include "core/css/cssom/CSSStyleValue.h"
 #include "core/css_property_names.h"
+#include "platform/bindings/ScopedPersistent.h"
 #include "platform/bindings/ScriptWrappable.h"
 #include "platform/bindings/TraceWrapperMember.h"
 #include "platform/bindings/TraceWrapperV8Reference.h"
@@ -39,8 +40,7 @@ class CSSLayoutDefinition final
 
   // This class represents an instance of the layout class defined by the
   // CSSLayoutDefinition.
-  class Instance final : public GarbageCollectedFinalized<Instance>,
-                         public TraceWrapperBase {
+  class Instance final : public GarbageCollectedFinalized<Instance> {
    public:
     Instance(CSSLayoutDefinition*, v8::Local<v8::Object> instance);
 
@@ -49,16 +49,12 @@ class CSSLayoutDefinition final
     bool Layout(const LayoutCustom&, FragmentResultOptions*);
 
     void Trace(blink::Visitor*);
-    void TraceWrappers(const ScriptWrappableVisitor*) const override;
-    const char* NameInHeapSnapshot() const override {
-      return "CSSLayoutDefinition::Instance";
-    }
 
    private:
     void ReportException(ExceptionState*);
 
     Member<CSSLayoutDefinition> definition_;
-    TraceWrapperV8Reference<v8::Object> instance_;
+    ScopedPersistent<v8::Object> instance_;
   };
 
   // Creates an instance of the web developer defined class. May return a