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/proxy.h b/webrtc/api/proxy.h
index 663e6c8..2df85c4 100644
--- a/webrtc/api/proxy.h
+++ b/webrtc/api/proxy.h
@@ -47,6 +47,8 @@
 #ifndef WEBRTC_API_PROXY_H_
 #define WEBRTC_API_PROXY_H_
 
+#include <memory>
+
 #include "webrtc/base/event.h"
 #include "webrtc/base/thread.h"
 
@@ -117,7 +119,7 @@
 
  private:
   void OnMessage(rtc::Message*) { proxy_->OnMessage(NULL); e_->Set(); }
-  rtc::scoped_ptr<rtc::Event> e_;
+  std::unique_ptr<rtc::Event> e_;
   rtc::MessageHandler* proxy_;
 };