[Extensions] Clean up a few more RenderFrameHost accessor calls.

For the transition to Service Worker-based background scripts, calls
to access the RenderFrameHost need to be changed, since there isn't
one available.

Bug: 950185
Change-Id: I3ba18a8922d05f27a66e1e05c88ae9b8bb094026
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598324
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657860}
2 files changed