Remove redundant field initialization in Java code.

base/android/java/src/org/chromium/base/NonThreadSafe.java

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.base;
 
 import org.chromium.base.annotations.SuppressFBWarnings;
 
 /**
  * NonThreadSafe is a helper class used to help verify that methods of a
  * class are called from the same thread.
  */
 public class NonThreadSafe {
-    private Long mThreadId = null;
+    private Long mThreadId;
 
     public NonThreadSafe() {
         ensureThreadIdAssigned();
     }
 
     /**
      * Changes the thread that is checked for in CalledOnValidThread. This may
      * be useful when an object may be created on one thread and then used
      * exclusively on another thread.
      */
@@ skipping 10 matching lines @@
     @SuppressFBWarnings("CHROMIUM_SYNCHRONIZED_METHOD")
     public synchronized boolean calledOnValidThread() {
         ensureThreadIdAssigned();
         return mThreadId.equals(Thread.currentThread().getId());
     }
 
     private void ensureThreadIdAssigned() {
         if (mThreadId == null) mThreadId = Thread.currentThread().getId();
     }
 }