Replace scoped_ptr with unique_ptr in webrtc/api/

But keep #including scoped_ptr.h in .h files, so as not to break
WebRTC users who expect those .h files to give them rtc::scoped_ptr.

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1930463002

Cr-Commit-Position: refs/heads/master@{#12530}
diff --git a/webrtc/api/webrtcsdp.cc b/webrtc/api/webrtcsdp.cc
index 10f9922..b86d903 100644
--- a/webrtc/api/webrtcsdp.cc
+++ b/webrtc/api/webrtcsdp.cc
@@ -13,7 +13,9 @@
 #include <ctype.h>
 #include <limits.h>
 #include <stdio.h>
+
 #include <algorithm>
+#include <memory>
 #include <string>
 #include <unordered_map>
 #include <vector>
@@ -2307,7 +2309,7 @@
         session_td.ice_mode, session_td.connection_role,
         session_td.identity_fingerprint.get());
 
-    rtc::scoped_ptr<MediaContentDescription> content;
+    std::unique_ptr<MediaContentDescription> content;
     std::string content_name;
     if (HasAttribute(line, kMediaTypeVideo)) {
       content.reset(ParseContentDescription<VideoContentDescription>(