Annotate libjingle_peerconnection_java with @Nullable.

Bug: webrtc:8881
Change-Id: Ida2ef6c003567d19529c21629c916ed40e8de3a6
Reviewed-on: https://webrtc-review.googlesource.com/63380
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22563}

sdk/android/api/org/webrtc/MediaStreamTrack.java

@@ -10,6 +10,8 @@
 
 package org.webrtc;
 
+import javax.annotation.Nullable;
+
 /** Java wrapper for a C++ MediaStreamTrackInterface. */
 @JNINamespace("webrtc::jni")
 public class MediaStreamTrack {
@@ -55,7 +57,7 @@ public class MediaStreamTrack {
   }
 
   /** Factory method to create an AudioTrack or VideoTrack subclass. */
-  static MediaStreamTrack createMediaStreamTrack(long nativeTrack) {
+  static @Nullable MediaStreamTrack createMediaStreamTrack(long nativeTrack) {
     if (nativeTrack == 0) {
       return null;
     }