Clear m_lastImageId for software compositor

Source/platform/graphics/Canvas2DLayerBridge.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 325 matching lines @@
     }
     if (!m_surfaceIsValid)
         setRateLimitingEnabled(false);
     return m_surfaceIsValid;
 }
 
 bool Canvas2DLayerBridge::prepareMailbox(blink::WebExternalTextureMailbox* outMailbox, blink::WebExternalBitmap* bitmap)
 {
     if (bitmap) {
         // Using accelerated 2d canvas with software renderer, which
-        // should only happen in tests that use fake graphics contexts.
-        // In this case, we do not care about producing any results for
-        // compositing.
+        // should only happen in tests that use fake graphics contexts
+        // or in Android WebView in software mode. In this case, we do
+        // not care about producing any results for this canvas.
         m_canvas->silentFlush();
+        m_lastImageId = 0;
         return false;
     }
     if (!isValid())
         return false;
 
     blink::WebGraphicsContext3D* webContext = context();
 
     // Release to skia textures that were previouosly released by the
     // compositor. We do this before acquiring the next snapshot in
     // order to cap maximum gpu memory consumption.
@@ skipping 129 matching lines @@
 Canvas2DLayerBridge::MailboxInfo::MailboxInfo(const MailboxInfo& other) {
     // This copy constructor should only be used for Vector reallocation
     // Assuming 'other' is to be destroyed, we transfer m_image ownership
     // rather than do a refcount dance.
     memcpy(&m_mailbox, &other.m_mailbox, sizeof(m_mailbox));
     m_image = const_cast<MailboxInfo*>(&other)->m_image.release();
     m_status = other.m_status;
 }
 
 }