p2p: Add HttpPeer to DownloadSource enumeration

This way the Installer.DownloadSourcesUsed metric conveys how often
p2p is used and new metrics Installer.SuccessfulMBsDownloadedFromHttpPeer
and Installer.TotalMBsDownloadedFromHttpPeer gives additional detail.

BUG=chromium:284714
TEST=modify unit tests to cover this case + unit tests pass

Change-Id: Ia4dff090091a282e1a184c2b18f8290749f5fdeb
Reviewed-on: https://chromium-review.googlesource.com/167913
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/constants.h b/constants.h
index 5b0ba8c..b221331 100644
--- a/constants.h
+++ b/constants.h
@@ -78,6 +78,7 @@
 typedef enum {
   kDownloadSourceHttpsServer, // UMA Binary representation: 0001
   kDownloadSourceHttpServer,  // UMA Binary representation: 0010
+  kDownloadSourceHttpPeer,    // UMA Binary representation: 0100
 
   // Note: Add new sources only above this line.
   kNumDownloadSources