count plan-b/unified-plan usage in SDP answers

the UMA stats currently do not count services like Hangouts that
have "complex" SDP with multiple tracks only in the answer, not in the
offer. Note that this changes the definition of the existing metric.

BUG=chromium:857004

Change-Id: Ib4520a82f7d94cdd4a307d32846e2d26a5f03b90
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/186701
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#32355}

pc/sdp_offer_answer.cc

diff --git a/pc/sdp_offer_answer.cc b/pc/sdp_offer_answer.cc
index 6d963ba..8b2d5a0 100644
--- a/pc/sdp_offer_answer.cc
+++ b/pc/sdp_offer_answer.cc
@@ -2009,8 +2009,9 @@
                  "Rollback not supported in Plan B"));
     return;
   }
-  if (desc->GetType() == SdpType::kOffer) {
-    // Report to UMA the format of the received offer.
+  if (desc->GetType() == SdpType::kOffer ||
+      desc->GetType() == SdpType::kAnswer) {
+    // Report to UMA the format of the received offer or answer.
     pc_->ReportSdpFormatReceived(*desc);
   }