Fix compilation issues of std::unique_ptr

webrtc/pc/videocapturertracksource.cc

Index: webrtc/pc/videocapturertracksource.cc
diff --git a/webrtc/pc/videocapturertracksource.cc b/webrtc/pc/videocapturertracksource.cc
index 2c64b42a079af86104482bf7e6ec17d4e116df36..216362b1888f6eb15fe9e9b224a29fe7f5110249 100644
--- a/webrtc/pc/videocapturertracksource.cc
+++ b/webrtc/pc/videocapturertracksource.cc
@@ -262,7 +262,7 @@ rtc::scoped_refptr<VideoTrackSourceInterface> VideoCapturerTrackSource::Create(
     const webrtc::MediaConstraintsInterface* constraints,
     bool remote) {
   RTC_DCHECK(worker_thread != NULL);
-  RTC_DCHECK(capturer != NULL);
+  RTC_DCHECK(capturer != nullptr);
   rtc::scoped_refptr<VideoCapturerTrackSource> source(
       new rtc::RefCountedObject<VideoCapturerTrackSource>(
           worker_thread, std::move(capturer), remote));
@@ -275,7 +275,7 @@ rtc::scoped_refptr<VideoTrackSourceInterface> VideoCapturerTrackSource::Create(
     std::unique_ptr<cricket::VideoCapturer> capturer,
     bool remote) {
   RTC_DCHECK(worker_thread != NULL);
-  RTC_DCHECK(capturer != NULL);
+  RTC_DCHECK(capturer != nullptr);
   rtc::scoped_refptr<VideoCapturerTrackSource> source(
       new rtc::RefCountedObject<VideoCapturerTrackSource>(
           worker_thread, std::move(capturer), remote));