Add MSID signaling compatibility for Unified Plan endpoints

This is intended to ensure compatibility between Plan B and
Unified Plan endpoints for the single audio - single video case.

If Unified Plan is the offerer, it will add a=msid and a=ssrc MSID
entries to its offer.
If Unified Plan is the answerer, it will use whatever MSID
signaling mechanism was used in the offer (either a=msid or
a=ssrc).

Bug: webrtc:7600
Change-Id: I6192dec19123fbb56f5d04540d2175c7fb30b9b6
Reviewed-on: https://webrtc-review.googlesource.com/44162
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21859}
diff --git a/pc/jsepsessiondescription.cc b/pc/jsepsessiondescription.cc
index 91840e3..ecd2ee2 100644
--- a/pc/jsepsessiondescription.cc
+++ b/pc/jsepsessiondescription.cc
@@ -273,7 +273,7 @@
   if (!description_ || !out) {
     return false;
   }
-  *out = SdpSerialize(*this, false);
+  *out = SdpSerialize(*this);
   return !out->empty();
 }