Remove most usages of EXECUTE_SCRIPT_USE_MANUAL_REPLY under
content/browser/webrtc

Removing reliance on EXECUTE_SCRIPT_USE_MANUAL_REPLY forces the
JavaScript to be written in terms of promises, which makes the code
less bug-prone since double-sends are systematically prevented (see
https://crrev.com/c/4318742 for an example bug that would have been
prevented by this).

There are two remaining usages under content/browser/webrtc, but they
are the entrypoints for a large number of tests, so I plan to rewrite
the corresponding javascript in followup CLs.

Bug: 1423407, 1422660
Change-Id: Ia2151f37e9ed5e206190a5ec35741225819ed2d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4327497
Auto-Submit: Chris Fredrickson <cfredric@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1116571}