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Add an url_loading_service (and factory) that will handle all url loading and will pass on UI actions to a delegate (here implemented in BrowserCoordinator, which in turn passes it on to BVC)

This will make chipping at URLLoader much easier.  The delegate api will change drastically in upcoming CL, as for now, it just reflects the current url load data structure and existing methods.

Bug: 907527
Change-Id: I6c72ad307d9e80249cac909149129dc593f9cbd1
Reviewed-on: https://chromium-review.googlesource.com/c/1442715
Commit-Queue: David Jean <djean@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628273}