Send and parse SCTP max-message-size in SDP

This also changes the default when no max-message-size is set
to the protocol defined value of 64K, and prevents messages
from being sent when they are too large to send.

Bug: webrtc:10358
Change-Id: Iacc1dd774d1554d9f27315378fbea6351300b5cc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135948
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27945}

pc/session_description.h

diff --git a/pc/session_description.h b/pc/session_description.h
index 27b781f..fe08e3a 100644
--- a/pc/session_description.h
+++ b/pc/session_description.h
@@ -498,7 +498,8 @@
   bool use_sctpmap_ = true;  // Note: "true" is no longer conformant.
   // Defaults should be constants imported from SCTP. Quick hack.
   int port_ = 5000;
-  int max_message_size_ = 256 * 1024;
+  // draft-ietf-mmusic-sdp-sctp-23: Max message size default is 64K
+  int max_message_size_ = 64 * 1024;
   std::unique_ptr<DataContentDescription> shim_;
 };