Remove usage of deprecagted V8 API from gin/

R=mkwst@chromium.org
TBR=abarth@chromium.org
BUG=324225

Review URL: https://codereview.chromium.org/94413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237750 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237750