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_unittest.cc b/omaha_response_handler_action_unittest.cc
index 9624b41..6fe48da 100644
--- a/omaha_response_handler_action_unittest.cc
+++ b/omaha_response_handler_action_unittest.cc
@@ -375,6 +375,9 @@
   OmahaRequestParams params(&mock_system_state);
   mock_system_state.set_request_params(&params);
 
+  EXPECT_CALL(*mock_system_state.mock_payload_state(),
+              SetUsingP2PForDownloading(true));
+
   string p2p_url = "http://9.8.7.6/p2p";
   params.set_p2p_url(p2p_url);
   params.set_use_p2p_for_downloading(true);