Enter the v8 isolate before using it in the proxy resolver

BUG=266838
R=svenpanne@chromium.org,eroman@chromium.org

Review URL: https://codereview.chromium.org/68763005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235125 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235125