Revert r17907 - Make it possible to add more than one piece of embedder data to isolates

Revert r17907 - 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

BUG=none
R=svenpanne@chromium.org
TBR=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/78093005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17915 ce2b1a6d-e550-0410-aec6-3dcde31c8c00