The Java Client should be aware of the readback status.

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

Index: content/public/android/java/src/org/chromium/content/browser/ContentReadbackHandler.java
diff --git a/content/public/android/java/src/org/chromium/content/browser/ContentReadbackHandler.java b/content/public/android/java/src/org/chromium/content/browser/ContentReadbackHandler.java
index ded0664c6296157b5f02f2260aacb8c15a4f64f0..227ec747fcbc7be4a6cec6acff4e6141e3cce161 100644
--- a/content/public/android/java/src/org/chromium/content/browser/ContentReadbackHandler.java
+++ b/content/public/android/java/src/org/chromium/content/browser/ContentReadbackHandler.java
@@ -27,7 +27,7 @@ public abstract class ContentReadbackHandler {
          * @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;
@@ -59,11 +59,11 @@ public abstract class ContentReadbackHandler {
 
 
     @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;
@@ -83,7 +83,7 @@ public abstract class ContentReadbackHandler {
     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();
@@ -103,7 +103,7 @@ public abstract class ContentReadbackHandler {
      */
     public void getCompositorBitmapAsync(WindowAndroid windowAndroid, GetBitmapCallback callback) {
         if (!readyForReadback()) {
-            callback.onFinishGetBitmap(null);
+            callback.onFinishGetBitmap(null, ReadbackResponse.READBACK_SURFACE_UNAVAILABLE);
             return;
         }
         ThreadUtils.assertOnUiThread();