[Extensions] Move frame registration to document commit time

Based on nasko@'s CL at
https://chromium-review.googlesource.com/c/chromium/src/+/5525999

When a navigation requires a new RenderFrameHost to be created for it,
the RenderFrameCreated observer method is called earlier in the
navigation process. There is potentially state that is incorrect at that
time, for example the SiteInstance for the navigation. When initializing
state related to the new document, a better point in time to do so is
the ReadyToCommitNavigation point in the timeline.

This CL moves the extensions WebContents observer code to use the
ReadyToCommit signal and does so behind a flag to ensure we can quickly
revert in case of regressions.

This also required a number of test updates, including:
* Properly waiting for a background page when installing an extension.
  Previously, we relied on the extension background page being
  registered in the process manager by the time the extension was added,
  which wasn't really guaranteed, but happened because we registered it
  earlier.
* Updating a BackgroundContents test to also wait for a hosted app's
  background contents to fully initialize.
* Updating a NaCl test to keep an extension background page alive long
  enough for a tab to load (which then keeps it alive indefinitely).

Bug: 334991035
Change-Id: Icc858c5c3b35970a4ead322c94f79c0f955afe46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6005549
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1381961}