Make it possible to add more than one piece of embedder data to isolates

This will allow for using gin and blink bindings in the same process

BUG=317398
R=svenpanne@chromium.org, dcarney@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/77913003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17907 ce2b1a6d-e550-0410-aec6-3dcde31c8c00