Let SessionDescription take ownership of MediaDescription

This documents in the API what is already true in the
implementation - that SessionDescription will eventually
delete MediaDescription objects passed to it.

The old API is preserved for backwards compatibility, but
marked as RTC_DEPRECATED.

Bug: webrtc:10701
Change-Id: I9a822b20cf3e58c5945fa51dbf6082960a332de8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/139880
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28144}

pc/peer_connection_bundle_unittest.cc

@@ -687,11 +687,13 @@ TEST_P(PeerConnectionBundleTest,
   ASSERT_GE(offer->description()->contents().size(), 2U);
   offer->description()
       ->contents()[0]
-      .description->mutable_streams()[0]
+      .media_description()
+      ->mutable_streams()[0]
       .ssrcs[0] = 1111222;
   offer->description()
       ->contents()[1]
-      .description->mutable_streams()[0]
+      .media_description()
+      ->mutable_streams()[0]
       .ssrcs[0] = 1111222;
   EXPECT_TRUE(
       caller->SetLocalDescription(CloneSessionDescription(offer.get())));