[Extensions] Finish freezing schema

BUG=604901
BUG=603725
BUG=591164

Review URL: https://codereview.chromium.org/1906593002

Cr-Commit-Position: refs/heads/master@{#388945}
  ExtensionImpl() : v8::Extension(kClassName, kScript) {}

  ExtensionImpl() : v8::Extension(kClassName, kScript) {}
