Introduce Nullable annotation.

Just adds the annotation to base_java for now to check that this does
not break any downstream targets.

Bug: webrtc:8881
Change-Id: I9425020e36be5e52447cec592a4474a9eb09b5bd
Reviewed-on: https://webrtc-review.googlesource.com/51960
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22526}

rtc_base/java/src/org/webrtc/ThreadUtils.java

@@ -16,13 +16,14 @@ import android.os.SystemClock;
 import java.util.concurrent.Callable;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
+import javax.annotation.Nullable;
 
 public class ThreadUtils {
   /**
    * Utility class to be used for checking that a method is called on the correct thread.
    */
   public static class ThreadChecker {
-    private Thread thread = Thread.currentThread();
+    @Nullable private Thread thread = Thread.currentThread();
 
     public void checkIsOnValidThread() {
       if (thread == null) {