Fix assert failure caused by ImageExtractor and optimize test

Source/platform/graphics/gpu/WebGLImageConversion.cpp

 // 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.
 
 #include "config.h"
 #include "platform/graphics/gpu/WebGLImageConversion.h"
 
 #include "platform/CheckedInt.h"
 #include "platform/graphics/ImageObserver.h"
 #include "platform/graphics/cpu/arm/WebGLImageConversionNEON.h"
@@ skipping 1566 matching lines @@
         // do AlphaDoUnmultiply if UNPACK_PREMULTIPLY_ALPHA_WEBGL is set to false.
         if (m_imageHtmlDomSource != HtmlDomVideo)
             m_alphaOp = AlphaDoUnmultiply;
     }
     if (!m_skiaImage)
         return false;
 
     m_imageSourceFormat = SK_B32_SHIFT ? DataFormatRGBA8 : DataFormatBGRA8;
     m_imageWidth = m_skiaImage->bitmap().width();
     m_imageHeight = m_skiaImage->bitmap().height();
-    if (!m_imageWidth || !m_imageHeight)
+    if (!m_imageWidth || !m_imageHeight) {
+        m_skiaImage.clear();
         return false;
+    }
     // Fail if the image was downsampled because of memory limits.
-    if (m_imageWidth != (unsigned)m_image->size().width() || m_imageHeight != (unsigned)m_image->size().height())
+    if (m_imageWidth != (unsigned)m_image->size().width() || m_imageHeight != (unsigned)m_image->size().height()) {
+        m_skiaImage.clear();
         return false;
+    }
     m_imageSourceUnpackAlignment = 0;
     m_skiaImage->bitmap().lockPixels();
     m_imagePixelData = m_skiaImage->bitmap().getPixels();
     return true;
 }
 
 unsigned WebGLImageConversion::getClearBitsByFormat(GLenum format)
 {
     switch (format) {
     case GL_ALPHA:
@@ skipping 164 matching lines @@
     }
 
     FormatConverter converter(width, height, sourceData, destinationData, srcStride, dstStride);
     converter.convert(sourceDataFormat, dstDataFormat, alphaOp);
     if (!converter.Success())
         return false;
     return true;
 }
 
 } // namespace WebCore