This CL cleans up the download target determination logic so that:

*  DownloadTargetDeterminer provides a reason for requesting user
   confirmation for a download path.

   The caller can thus use platform specific logic to handle the
   confirmation request. Chromium on Android doesn't always prompt the
   user for a download location, and hence needs to know *why* a
   confirmation is requested in order to decide how to proceed.

*  DownloadTargetDeterminer allows the caller to return a finer grained
   result than a boolean following a confirmation request.

   Just knowing whether the user confirmed or rejected is insufficient.
   On Android, for example, there are cases where the platform specific
   code will automatically accept a confirmation request raised by the
   DownloadTargetDeterminer. In such cases, it is important to note
   whether the result indicates explicit user consent or not, which is a
   signal used elsewhere for determining download security.

*  DownloadPathReservationTracker provides a detailed result after
   completing target determination.

   The extra detail allows DownloadTargetDeterminer to indicate *why*
   the target determination failed rather than just the fact that it
   failed. This result can then be translated to an interrupt reason.

Consequently, the download target conflict resolution logic can now be
moved entirely into ChromeDownloadManagerDelegate. Previously the logic
was split between DownloadTargetDeterminer,
DownloadPathReservationTracker, and ChromeDownloadManagerDelegate on
Android.

BUG=334474

Change-Id: Id3e40bcb4653f259dab1ae96645c230a2a45e06c
Reviewed-on: https://chromium-review.googlesource.com/465566


Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#464084}