[Extensions] Don't allow extensions to inject scripts into extension pages

Don't allow extensions to inject scripts into other extension pages, since this
is a security risk. This was meant to be addressed, but there was an incorrect
early-return. Also add a regression test.

BUG=529682

Review URL: https://codereview.chromium.org/1335083004

Cr-Commit-Position: refs/heads/master@{#348707}
12 files changed