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As part of preparing for multi-window, the URLLoadingService needs
to be migrated to be a BrowserAgent instead of a KeyedService. That
change is itself fairly large, but as an additional complication
URLLoadingService is used in downstream code.

This CL is the first in a series of changes needed to complete this
refactor:

1. Add a new provider API upstream that passes both a Browser and
   temporary URL loading shim to CreateLogoVendor.
2. Implement this new API downstream and add support for initializing
   the LogoController with both Browser and the loading shim. Use the
   loading shim for loading URLs if it is present.
3. Upstream, call the new CreateLogoVendor() API.
4. Downstream, remove support for the old CreateLogoVendor() API and
   update LogoController to only be initialized with a Browser and the
   shim loader. Remove downstream use of the URLLoadingService.
5. Upstream, refactor URLLoadingService to be URLLoadingBrowserAgent,
   including updating the upstream implementation of the loading shim
   passed downstream in CreateLogoVendor().
6. Upstream, add a new provider API that no longer passes the loading
   shim into CreateLogoVendor().
7. Downstream, remove use of the loading shim from LogoController and
   have it fetch the URLLoadingBrowserAgent from its Browser parameter.
   Implement the new shim-less CreateLogoVendor() API. Remove the
   implementation of the shimmed API.
8. Upstream, remove the loading shim protocol and the old shimmed
   CreateLogoVendor() API. Call only the new shimless API.

Specifically, this CL creates a shim URLLoadingBridge protocol so that
the implementation of the actual URL load can be changed upstream in
a single CL upstream (step 5 in the process above). At the same time,
a Browser object is also passed into the provider so that after the
loading service is changed, it can be fetched (via Browser) in the
downstream code.

This CL doesn't introduce any implementation of the shim protocol; that
will be in step 3.

Bug: 1046374
Change-Id: Ic2388c0f9680dabc8cb10349255cc9ab281cccbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089687Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747748}