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This CL introduces a new enum that track the download source.

Currently it lives in content/public/browser, and passed in from
content::DownloadUrlParameters. In components/download, there is a one
to one mapping enum for usage in download in progress db, which can not
depend on content/public.

This enum is supposed to used in UMA as suffix for several key metrics
and UKM as Components, and saved in the new in progress level db.

Bug: 786482
Change-Id: I19a2f959eb9e7b0787087cad6c9519acbde13281
Reviewed-on: https://chromium-review.googlesource.com/777548
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519928}