Fix extension bindings injection for iframes

For iframes, we don't want to use the source url for determining the
associated extension because it starts out with an about:blank context
that is scriptable by its parent.

BUG=573131

Review-Url: https://codereview.chromium.org/2151693002
Cr-Commit-Position: refs/heads/master@{#407214}
9 files changed