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Moves AppURLLoadingServiceDelegate from Main to Scene Controller.
* adds SceneControllerGuts temporary interface to expose scene-level
internals to MainController while the code is moved.

Bug: None
Change-Id: I0c7dbe077a94ff5b9c598c401c9c7340cc8f969c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1983226
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730151}