Use gin::IsolateHolder as one and only holder of v8::Isolates

BUG=321631
R=marja@chromium.org

Review URL: https://codereview.chromium.org/110153002

git-svn-id: svn://svn.chromium.org/blink/trunk@163437 bbb929c8-8fbe-4397-9dbb-9b2b20218538