The Java Client should be aware of the readback status.

content/public/android/java/src/org/chromium/content/browser/ContentReadbackHandler.java

 // Copyright 2014 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.content.browser;
 
 import android.graphics.Bitmap;
 import android.graphics.Rect;
 import android.util.SparseArray;
 
 import org.chromium.base.CalledByNative;
 import org.chromium.base.JNINamespace;
 import org.chromium.base.ThreadUtils;
 import org.chromium.ui.base.WindowAndroid;
 
 /**
  * A class for reading back content.
  */
 @JNINamespace("content")
 public abstract class ContentReadbackHandler {
     /**
      * A callback interface for content readback into a bitmap.
      */
     public static interface GetBitmapCallback {
         /**
          * Called when the content readback finishes.
          * @param bitmap     The {@link Bitmap} of the content.  Null will be passed for readback
          *                   failure.

[AKV, 2015/04/01 14:49:24]

Please update doxygn description corresponds to newly added response parameter.

          */
-        public void onFinishGetBitmap(Bitmap bitmap);
+        public void onFinishGetBitmap(Bitmap bitmap, int response);
     }
 
     private int mNextReadbackId = 1;
     private SparseArray<GetBitmapCallback> mGetBitmapRequests;
 
     private long mNativeContentReadbackHandler;
 
     /**
      * Creates a {@link ContentReadbackHandler}.
      */
@@ skipping 11 matching lines @@
     /**
      * Should be called when the ContentReadackHandler is not needed anymore.
      */
     public void destroy() {
         if (mNativeContentReadbackHandler != 0) nativeDestroy(mNativeContentReadbackHandler);
         mNativeContentReadbackHandler = 0;
     }
 
 
     @CalledByNative
-    private void notifyGetBitmapFinished(int readbackId, Bitmap bitmap) {
+    private void notifyGetBitmapFinished(int readbackId, Bitmap bitmap, int response) {
         GetBitmapCallback callback = mGetBitmapRequests.get(readbackId);
         if (callback != null) {
             mGetBitmapRequests.delete(readbackId);
-            callback.onFinishGetBitmap(bitmap);
+            callback.onFinishGetBitmap(bitmap, response);

[AKV, 2015/04/01 14:49:24]

You may have to incorporate same changes in
ContentViewReadbackTest->onFinishGetBitmap() function as well to make
ContentShell test bot happy. Please take care of that as well.

         } else {
             // readback Id is unregistered.
             assert false : "Readback finished for unregistered Id: " + readbackId;
         }
     }
 
     /**
      * Asynchronously, generate and grab a bitmap representing what is currently on the screen
      * for {@code view}.
      *
      * @param scale The scale that should be applied to the content.
      * @param srcRect  A subrect of the original content to capture. If this is empty, it will grab
      *                 the whole surface.
      * @param view  The {@link ContentViewCore} to grab the bitmap from.
      * @param callback The callback to be executed after readback completes.
      */
     public void getContentBitmapAsync(float scale, Rect srcRect, ContentViewCore view,
             GetBitmapCallback callback) {
         if (!readyForReadback()) {
-            callback.onFinishGetBitmap(null);
+            callback.onFinishGetBitmap(null, ReadbackResponse.READBACK_SURFACE_UNAVAILABLE);
             return;
         }
         ThreadUtils.assertOnUiThread();
 
         int readbackId = mNextReadbackId++;
         mGetBitmapRequests.put(readbackId, callback);
         nativeGetContentBitmap(mNativeContentReadbackHandler, readbackId, scale,
                 Bitmap.Config.ARGB_8888, srcRect.top, srcRect.left, srcRect.width(),
                 srcRect.height(), view);
     }
 
     /**
      * Asynchronously, grab a bitmap of the current browser compositor root layer.
      *
      * @param windowAndroid The window that hosts the compositor.
      * @param callback      The callback to be executed after readback completes.
      */
     public void getCompositorBitmapAsync(WindowAndroid windowAndroid, GetBitmapCallback callback) {
         if (!readyForReadback()) {
-            callback.onFinishGetBitmap(null);
+            callback.onFinishGetBitmap(null, ReadbackResponse.READBACK_SURFACE_UNAVAILABLE);
             return;
         }
         ThreadUtils.assertOnUiThread();
 
         int readbackId = mNextReadbackId++;
         mGetBitmapRequests.put(readbackId, callback);
         nativeGetCompositorBitmap(mNativeContentReadbackHandler, readbackId,
                 windowAndroid.getNativePointer());
     }
 
     /**
      * Implemented by the owner of this class to signal whether readback is possible or not.
      * @return Whether readback is possible or not.
      */
     protected abstract boolean readyForReadback();
 
     private native long nativeInit();
     private native void nativeDestroy(long nativeContentReadbackHandler);
     private native void nativeGetContentBitmap(long nativeContentReadbackHandler, int readbackId,
             float scale, Bitmap.Config config, float x, float y, float width, float height,
             Object contentViewCore);
     private native void nativeGetCompositorBitmap(long nativeContentReadbackHandler,
             int readbackId, long nativeWindowAndroid);
 }