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/omaha_response_handler_action.cc b/omaha_response_handler_action.cc
index 7df4c8d..8e77afa 100644
--- a/omaha_response_handler_action.cc
+++ b/omaha_response_handler_action.cc
@@ -77,6 +77,7 @@
               << " with local URL " << params->p2p_url()
               << " since p2p is enabled.";
     install_plan_.download_url = params->p2p_url();
+    system_state_->payload_state()->SetUsingP2PForDownloading(true);
   }
 
   // Fill up the other properties based on the response.