Pass browser user agent to android DownloadManager if no override UA is present.

Pass browser user agent to android DownloadManager if no override UA is present.

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
Committed: https://crrev.com/7c43b0968c2063ddeb6fdbdddb52dce9209f31bb
Cr-Commit-Position: refs/heads/master@{#330861}

[qinmin, 2015-05-20 23:35:43]

qinmin@chromium.org changed reviewers:
+ tedchoc@chromium.org

[qinmin, 2015-05-20 23:35:44]

PTAL

[Ted C, 2015-05-20 23:38:30]

lgtm

[qinmin, 2015-05-20 23:40:28]

The CQ bit was checked by qinmin@chromium.org

[commit-bot: I haz the power, 2015-05-20 23:41:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1145303004/1

[commit-bot: I haz the power, 2015-05-21 00:35:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-21 00:36:41]

Patchset 1 (id:??) landed as
https://crrev.com/7c43b0968c2063ddeb6fdbdddb52dce9209f31bb
Cr-Commit-Position: refs/heads/master@{#330861}