Remove unnecessary bits in MojoWebUIController after migration to BIB

Now that all the subclasses of MojoWebUIController have been migrated to
the BrowserInterfaceBroker, we need to update the high-level documentation
of this class, stop inheriting from WebContentsObserver and remove the
bits that are not needed anymore: the local instance of BinderRegistry as
well as the OnInterfaceRequestFromFrame and AddHandlerToRegistry methods.

Bug: 936482, 1002647
Change-Id: Id49d9b0c13a7cf971d0fb4040c7632b36f50cc0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1972844
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728867}
2 files changed