Do a null check on BrowserContext in DownloadManagerService

DownloadManagerService may start too early before full browser is started.
In this case, downloadItem doesn't have a BrowserContext object assocuated,

BUG=695115

Change-Id: Ie7dcc88bb63cdfc8af0d0e4d4670e808e22f175e
Reviewed-on: https://chromium-review.googlesource.com/1140519
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575863}
1 file changed