Improve Setter of SVG attribute generation code.

Currently, following Setter code is generated.
in V8TestObject.cpp
imp->setMutablePoint(WTF::getPtr(v));

It forces us to implement following code.
void TestObject::setMutablePoint(SVGPropertyTearOff<SVGPoint>* point);

It has two big issues.
1. SVGPropertyTearOff<T>* can be null.
For example, in javascript
testObject.mutablePoint = "Not SVGPoint";
2. The cpp code is so ugly, because the cpp code must know SVGPropertyTearOff.

This CL improves SVG attribute Setter generation.
When SVGPropertyTearOff<T>* is null, generated code throws TypeError exception.

BUG=301660

Review URL: https://codereview.chromium.org/25037006

git-svn-id: svn://svn.chromium.org/blink/trunk@159231 bbb929c8-8fbe-4397-9dbb-9b2b20218538