[Extensions] Create WebContentsObservers with ExtensionsBrowserClient

There are different variations of the ExtensionWebContentsObserver based
on the embedder (i.e., one for Shell and one for Chrome). Instead of
having individual callers know about this distinction, instantiate these
WebContentsObservers through the ExtensionsBrowserClient by introducing
ExtensionsBrowserClient::CreateExtensionWebContentsObserver(). The
ExtensionsBrowserClient already had knowledge of these for the
related GetExtensionWebContentsObserver() method, and this allows more
code to be shared across different build configurations.

This CL should have no behavior change.

Bug: 356905053
Change-Id: I75acd6cb2a0a151d193c596d97509c5b26700910
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5825636
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1350962}