Allowing global references to be read through a proxy results in cross-origin information leaks. The ES6 spec currently does not mitigate this in any way. This CL adds a workaround that's easy for V8: throw whenever an unresolved reference would result in a proxy trap to be fired. I'm landing this so we can move forwards with staging proxies without putting users of --harmony at risk.

BUG=chromium:399951
LOG=n

Review URL: https://codereview.chromium.org/1529303003

Cr-Commit-Position: refs/heads/master@{#32949}