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user-agent may not present in URlRequest->extra_request_headers() if the field is not overriden.
In this case, we should pass browser's UA to the android download manager.
We have done something similar in MediaPlayerBridge too.

BUG=488444

Review URL: https://codereview.chromium.org/1145303004

Cr-Commit-Position: refs/heads/master@{#330861}