IDL: Make SerializedScriptValue arguments less of a special case

Remove the special handling of SerializedScriptValue arguments from the
methods.cpp template, and instead fix the already existing handling in
v8_types.py.

The conversion uses an ExceptionState object to report exceptions, and is
now handled like any other conversion that does that.

BUG=413257

Review URL: https://codereview.chromium.org/600863002

git-svn-id: svn://svn.chromium.org/blink/trunk@182613 bbb929c8-8fbe-4397-9dbb-9b2b20218538