[Offline Pages] Close race condition of multiple StartProcessing callers.

chrome/android/java/src/org/chromium/chrome/browser/offlinepages/BackgroundOfflinerTask.java

 // Copyright 2016 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.chrome.browser.offlinepages;
 
 import android.content.Context;
 import android.os.Bundle;
 
 import org.chromium.base.ApplicationStatus;
@@ skipping 82 matching lines @@
              */
             @Override
             public void onResult(Boolean result) {
                 // Release the wake lock.
                 Log.d(TAG, "onProcessingDone");
                 waiter.onWaitDone();
             }
         };
 
         // Pass the activation on to the bridge to the C++ RequestCoordinator.
-        mBridge.startProcessing(deviceConditions, callback);
+        if (!mBridge.startProcessing(deviceConditions, callback)) {
+            // Processing not started currently. Let callback know.
+            callback.onResult(false);
+        }
     }
 }