Make toV8(ScriptValue) never return an empty handle

I'm making toV8() never return an empty handle.
This CL replaces the return value of toV8(ScriptValue) with v8::Undefined,
but it won't change any web-exposed behavior because V8 interprets
a returned empty handle as v8::Undefined().

BUG=625512

Review-Url: https://codereview.chromium.org/2280893002
Cr-Commit-Position: refs/heads/master@{#414688}

third_party/WebKit/Source/bindings/core/v8/ToV8.h

@@ -185,17 +185,19 @@ inline v8::Local<v8::Value> toV8(const ToV8UndefinedGenerator&  value, v8::Local
 
 // ScriptValue
 
-inline v8::Local<v8::Value> toV8(const ScriptValue& value, v8::Local<v8::Object> creationContext, v8::Isolate*)
+inline v8::Local<v8::Value> toV8(const ScriptValue& value, v8::Local<v8::Object> creationContext, v8::Isolate* isolate)
 {
+    if (value.isEmpty())
+        return v8::Undefined(isolate);
     return value.v8Value();
 }
 
 // Dictionary
 
-inline v8::Local<v8::Value> toV8(const Dictionary& value, v8::Local<v8::Object> creationContext, v8::Isolate*)
+inline v8::Local<v8::Value> toV8(const Dictionary& value, v8::Local<v8::Object> creationContext, v8::Isolate* isolate)
 {
-    RELEASE_NOTREACHED();
-    return v8::Local<v8::Value>();
+    NOTREACHED();
+    return v8::Undefined(isolate);
 }
 
 inline v8::Local<v8::Value> toV8(const IDLDictionaryBase& value, v8::Local<v8::Object> creationContext, v8::Isolate* isolate)