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

Reland 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.

Over r17907, I changed the order of fields in Isolate to be stable across different platforms, since the ABI defined packing is not the same on
all targets, and I initialize the embedder data field in Isolate.

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

Review URL: https://codereview.chromium.org/78453002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17935 ce2b1a6d-e550-0410-aec6-3dcde31c8c00