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This CL treats extension downloads from untrusted sources as regular
downloads. That means these downloads will be prompted, show up in
downloads page and download shelf as normal downloads.

BUG=756825

Change-Id: I515bba2fa0a64198a56a06ce38864ef59ad18c9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761374Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689091}