Deduplicate BrowserContext's ResourceContext

This seems to be an empty class meant to be removed at some point.
Today, this is only a class supporting `SupportsUserData`.

I observed every content/ embedder was required to provide one, and none
of them was really providing some kind of customization.

This patch merge all the different embedders implementations into
the content/ implementation. This has the
side effect of helping linnan@chromium.org to remove a dangling pointer.
This is an alternative to her patch from:
https://chromium-review.googlesource.com/c/chromium/src/+/5132493

Bug: chromium:1471070, chromium:908955
Change-Id: I4938957822d0a77be642aef3098faaa87616295f
Fixed: 1471070
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5139789
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Owners-Override: John Abd-El-Malek <jam@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1240500}