Use |nullptr| instead of |0| numeric literal.

Bug: 1080832
Change-Id: I95435ef51b4231a2f08368af722a6bff6b8c4c6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208542Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771035}

third_party/blink/renderer/modules/mediarecorder/media_recorder_handler_unittest.cc

@@ -192,7 +192,7 @@ class MediaRecorderHandlerTest : public TestWithParam<MediaRecorderTestParams>,
   // For generating test AudioBuses
   media::SineWaveAudioSource audio_source_;
 
-  MockMediaStreamVideoSource* video_source_ = 0;
+  MockMediaStreamVideoSource* video_source_ = nullptr;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MediaRecorderHandlerTest);