[Extensions] Harden against bindings interception

      There's more we can do but this is a start.

BUG=590275
BUG=590118

Review URL: https://codereview.chromium.org/1748943002

Cr-Commit-Position: refs/heads/master@{#378621}
(cherry picked from commit 75b803b1c81ed9fa5513cbff550232b4fb915e7b)

Review URL: https://codereview.chromium.org/1787433002 .

Cr-Commit-Position: refs/branch-heads/2661@{#186}
Cr-Branched-From: ef6f6ae5e4c96622286b563658d5cd62a6cf1197-refs/heads/master@{#378081}