Pass webview's useragent when downloading via context menu

Currently null is passed as useragent to the Download Manager when
downloading via Browser context menu (causes the Download
Provider to use an empty useragent HTTP header).
This commit simply passes the webview's useragent instead.

This applies to the usecase when choosing save image/link
via the context menu (long press).

Change-Id: Ie9cc5e81630d0dcefe4708980146cc5ed867e4ad
1 file changed