Only cancel in-progress downloads when closing the last Browser window.

BUG=320592

Review URL: https://codereview.chromium.org/82053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236705 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236705