Changed defaults for CreateAnswer in non-constraint mode

This CL also adds control flag in webrtcsession_unittests
that says whether to prefer constraints APIs or non-constraints APIs, and uses it in the test that was needed
to uncover the bug.

BUG=webrtc:4906

Review URL: https://codereview.webrtc.org/1775033002

Cr-Commit-Position: refs/heads/master@{#11947}
diff --git a/webrtc/api/webrtcsession_unittest.cc b/webrtc/api/webrtcsession_unittest.cc
index 87ff927..2c9a99b 100644
--- a/webrtc/api/webrtcsession_unittest.cc
+++ b/webrtc/api/webrtcsession_unittest.cc
@@ -518,7 +518,7 @@
   void GetOptionsForOffer(
       const PeerConnectionInterface::RTCOfferAnswerOptions& rtc_options,
       cricket::MediaSessionOptions* session_options) {
-    ASSERT_TRUE(ExtractMediaSessionOptions(rtc_options, session_options));
+    ASSERT_TRUE(ExtractMediaSessionOptions(rtc_options, true, session_options));
 
     AddStreamsToOptions(session_options);
     if (rtc_options.offer_to_receive_audio ==