[webNavigation] only observe redirects for frames

Added a test that e.g. a redirects for a download does not trigger the
DCHECK that we know about the frame.

The if statement in web_navigation_api.cc used to be in place but was
accidentally removed in r241514

BUG=329456
TEST=WebNavigationApiTest.Download
R=battre@chromium.org

Review URL: https://codereview.chromium.org/98903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241613 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241613