bindings: Introduces ScriptWrappableBase to provide the internal pointer.

Introduces ScriptWrappableBase class in order to provide the internal pointer independent from V8Type.

The end to end change will be http://crrev.com/420233002

BUG=235436

Review URL: https://codereview.chromium.org/422873002

git-svn-id: svn://svn.chromium.org/blink/trunk@179098 bbb929c8-8fbe-4397-9dbb-9b2b20218538