Switching some interfaces to use std::unique_ptr<>.

This helps show where ownership is transfered between objects.

Specifically, this CL wraps cricket::VideoCapturer, MediaEngineInterface
and DataEngineInterface in unique_ptr.

BUG=None
TBR=magjed@webrtc.org

Review-Url: https://codereview.webrtc.org/2685093002
Cr-Commit-Position: refs/heads/master@{#16548}

webrtc/api/peerconnectionfactoryproxy.h

@@ -25,6 +25,11 @@ namespace webrtc {
 // are called on is an implementation detail.
 BEGIN_SIGNALING_PROXY_MAP(PeerConnectionFactory)
   PROXY_SIGNALING_THREAD_DESTRUCTOR()
+  // Use the overloads of CreateVideoSource that take raw VideoCapturer
+  // pointers from PeerConnectionFactoryInterface.
+  // TODO(deadbeef): Remove this using statement once those overloads are
+  // removed.
+  using PeerConnectionFactoryInterface::CreateVideoSource;
   PROXY_METHOD1(void, SetOptions, const Options&)
   PROXY_METHOD5(rtc::scoped_refptr<PeerConnectionInterface>,
                 CreatePeerConnection,
@@ -48,11 +53,11 @@ BEGIN_SIGNALING_PROXY_MAP(PeerConnectionFactory)
                 const cricket::AudioOptions&)
   PROXY_METHOD2(rtc::scoped_refptr<VideoTrackSourceInterface>,
                 CreateVideoSource,
-                cricket::VideoCapturer*,
+                std::unique_ptr<cricket::VideoCapturer>,
                 const MediaConstraintsInterface*)
   PROXY_METHOD1(rtc::scoped_refptr<VideoTrackSourceInterface>,
                 CreateVideoSource,
-                cricket::VideoCapturer*)
+                std::unique_ptr<cricket::VideoCapturer>)
   PROXY_METHOD2(rtc::scoped_refptr<VideoTrackInterface>,
                 CreateVideoTrack,
                 const std::string&,