[chromium_build_stats] set Content Header

gz file can be downloaded.
https://19926-7566ce9-tainted-tikuta-content-dot-chromium-build-stats.appspot.com/file/2019/01/11/swarm1887-c4/compiler_proxy.swarm1887-c4.chrome-bot.log.INFO.20190110-185154.19523.gz

Works fine for chromeos too.
https://19926-7566ce9-tainted-tikuta-content-dot-chromium-build-stats.appspot.com/file/2019/01/09/swarm-cros-618/compiler_proxy.swarm-cros-618.chrome-bot.log.INFO.20190109-225358.6391.gz
https://19926-7566ce9-tainted-tikuta-content-dot-chromium-build-stats.appspot.com/file/2019/01/09/swarm-cros-618/compiler_proxy.swarm-cros-618.chrome-bot.log.INFO.20190109-212332.1035.gz

Bug: 920890
Change-Id: Ib724a7205db800d13fe7cde48e53adc9c3f3361a
Reviewed-on: https://chromium-review.googlesource.com/c/1406468
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Reviewed-by: Fumitoshi Ukai <ukai@chromium.org>
Reviewed-by: Shinya Kawanaka <shinyak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#19923}
Cr-Mirrored-Commit: a432dbf721c20ef2ffefa0684f604c3dc316e838
diff --git a/go/src/infra/appengine/chromium_build_stats/app/file.go b/go/src/infra/appengine/chromium_build_stats/app/file.go
index 2767fc6..df721b2 100644
--- a/go/src/infra/appengine/chromium_build_stats/app/file.go
+++ b/go/src/infra/appengine/chromium_build_stats/app/file.go
@@ -54,6 +54,9 @@
 	}
 	defer reader.Close()
 
+	w.Header().Set("Content-Type", reader.ContentType())
+	w.Header().Set("Content-Encoding", reader.ContentEncoding())
+
 	_, err = io.Copy(w, reader)
 	if err != nil {
 		log.Errorf(ctx, "failed to copy %s: %v", path, err)