Downloads: separate BrowserProxy from Mojo classes for clarity

R=calamity@chromium.org,dpapad@chromium.org
BUG=855259

Change-Id: I8e3ea89831ce9c39ac05afa0d4f18c1c51c3a8cb
Reviewed-on: https://chromium-review.googlesource.com/c/1364233
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614570}
7 files changed