[Downloads location] Create a preference for downloads location.

This CL is part of the greater effort to build out functionality to
allow users to change their default download storage location. This is
the first step, surfacing the option to change download location in the
preferences pane. This is not fully functional (the directory selector
is not yet implemented) and is hidden behind a flag.

Bug: 792775
Change-Id: Ic57aa2e4f61c83861e5711c2fe0b48737217904e
Reviewed-on: https://chromium-review.googlesource.com/818205
Commit-Queue: Joy Ming <jming@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523258}
8 files changed