Add android download controller.

      The android download controller is responsible for sending GET
downloads to the JAVA side (to be handled by Android DownloadManager).

 - It also prepares all the info needed for the download (cookies etc)
 - POST downloads are handled natively, the download controller can be
   used to pass start/completed notifications to the JAVA side.

BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10449017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@139600 0039d316-1c4b-4281-b951-d872f2087c98