content: make DesktopMediaID environment-agnostic

This change breaks the direct dependency between Aura and DesktopMediaID, in
preparation for bringing DesktopMediaPickerViews to Mac. Specifically, the
DesktopMediaID functions to register windows by ID now take gfx::NativeWindow
instead of aura::Window*, and the Aura-specific implementation of those
functions is moved to its own file. Other references to Aura in the public API
of DesktopMediaID are renamed to reflect that they may operate on any type of
NativeWindow as well.

Bug: 726005
Change-Id: I57eddd1434294af99f9eae11fea4b56b7d3128b1
Reviewed-on: https://chromium-review.googlesource.com/c/1475006
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633369}
20 files changed