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}
diff --git a/sdk/android/api/org/webrtc/ScreenCapturerAndroid.java b/sdk/android/api/org/webrtc/ScreenCapturerAndroid.java
index 260d7a3..1b5dde7 100644
--- a/sdk/android/api/org/webrtc/ScreenCapturerAndroid.java
+++ b/sdk/android/api/org/webrtc/ScreenCapturerAndroid.java
@@ -19,6 +19,7 @@
 import android.media.projection.MediaProjection;
 import android.media.projection.MediaProjectionManager;
 import android.view.Surface;
+import javax.annotation.Nullable;
 
 /**
  * An implementation of VideoCapturer to capture the screen content as a video stream.
@@ -44,13 +45,13 @@
 
   private int width;
   private int height;
-  private VirtualDisplay virtualDisplay;
-  private SurfaceTextureHelper surfaceTextureHelper;
-  private CapturerObserver capturerObserver;
+  @Nullable private VirtualDisplay virtualDisplay;
+  @Nullable private SurfaceTextureHelper surfaceTextureHelper;
+  @Nullable private CapturerObserver capturerObserver;
   private long numCapturedFrames = 0;
-  private MediaProjection mediaProjection;
+  @Nullable private MediaProjection mediaProjection;
   private boolean isDisposed = false;
-  private MediaProjectionManager mediaProjectionManager;
+  @Nullable private MediaProjectionManager mediaProjectionManager;
 
   /**
    * Constructs a new Screen Capturer.