Added a Isolate* parameter to Serializer::enabled().

This parameter will soon be used when a few pseudo-classes like
Serializer are turned into real classes. The current CL is already big
enough, untying our Gordian knot called "startup" will continue...

BUG=359977
LOG=y
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/260003006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21083 ce2b1a6d-e550-0410-aec6-3dcde31c8c00