Add path of the file with fetched data in external data policy pipeline.

It will be mostly used by CloudExternalDataPolicyObserver::Delegate and DeviceCloudExternalDataPolicyObserver::Delegate, which are the end point of the pipeline.
This will be used to migrate DeviceWallpaperImage policy from custom downloader to existing pipeline in upcoming CL.

Bug: 814364
Change-Id: I4f87823cb5495c6c880493a76719b94dfeb8bb99
Reviewed-on: https://chromium-review.googlesource.com/c/1403128
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Nikita Podguzov <nikitapodguzov@google.com>
Cr-Commit-Position: refs/heads/master@{#624398}
28 files changed