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In the delegate proxy refactor (crrev.com/1422158), some if statements
where changed to DCHECKs. When aborted, some of the Download Service
callbacks can still be received due to race conditions.

This CL switches the DCHECKs back to if statements.

Bug: 850512
Change-Id: I4154070635d5434819c07b394fbfe323ca9fdb9e
Reviewed-on: https://chromium-review.googlesource.com/c/1456063Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629554}