fake_omaha: fix server address

Since apache2 will be forwarding update requests to nginx, we can simply
use X-Forwarded-Host header value to generate the update url.

BUG=chromium:1078188
TEST=Manually tested. Details can be found at
http://gpaste/5025076468514816

Change-Id: I120b8f03cff4b48a9387543749d2db3059dc97ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2293143
Tested-by: Sanika Kulkarni <sanikak@chromium.org>
Commit-Queue: Sanika Kulkarni <sanikak@chromium.org>
Auto-Submit: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
1 file changed