Create gin::IsolateHolders for blink managed Isolates.

This also introduces a compile time dependency on gin. So far, we
only used constants from public headers

BUG=321631
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/106413002

git-svn-id: svn://svn.chromium.org/blink/trunk@163290 bbb929c8-8fbe-4397-9dbb-9b2b20218538