ScriptString should not inherit from ScriptValue

- This CL is a preparation for making ScriptValue understand the ScriptState from which the ScriptValue is created.

- Since ScriptString should not be associated with ScriptState (String is managed per-Isolate, not per-context; ScriptString is used in places where we don't have ScriptState), this CL breaks the inheritance from ScriptValue to ScriptString.

BUG=357144

Review URL: https://codereview.chromium.org/261523004

git-svn-id: svn://svn.chromium.org/blink/trunk@173081 bbb929c8-8fbe-4397-9dbb-9b2b20218538