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}
(cherry picked from commit 91f655b19888da3f86b57ad8c548da93e7b9aba4)

Review URL: https://codereview.chromium.org/2183443002 .

Cr-Commit-Position: refs/branch-heads/2785@{#331}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}
9 files changed