fixing bug in 2D context painted area pixel count computation ...

chrome/renderer/webplugin_delegate_pepper.cc

 // Copyright (c) 2010 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.
 
 #define PEPPER_APIS_ENABLED 1
 
 #include "build/build_config.h"
 #if defined(OS_WIN)
 #include <vsstyle.h>
 #endif
@@ skipping 462 matching lines @@
       return NPERR_INVALID_PARAM;
     Graphics2DDeviceContext* ctx = graphic2d_contexts_.Lookup(
         reinterpret_cast<intptr_t>(context->reserved));
     if (!ctx)
       return NPERR_INVALID_PARAM;
     *value = reinterpret_cast<intptr_t>(ctx->transport_dib());
     return NPERR_NO_ERROR;
   } else if (state == NPExtensionsReservedStateSharedMemoryChecksum) {
     if (!context)
       return NPERR_INVALID_PARAM;
-    int32 row_count = context->dirty.bottom - context->dirty.top + 1;
-    int32 stride = context->dirty.right - context->dirty.left + 1;
-    size_t length = row_count * stride * sizeof(uint32);
+    // Bytes per pixel.
+    static const int kBytesPixel = 4;
+    int32 row_count = context->dirty.bottom - context->dirty.top;
+    int32 stride = context->dirty.right - context->dirty.left;
+    size_t length = row_count * stride * kBytesPixel;
     MD5Digest md5_result;   // 128-bit digest
     MD5Sum(context->region, length, &md5_result);
     std::string hex_md5 = MD5DigestToBase16(md5_result);
     // Return the least significant 8 characters (i.e. 4 bytes)
     // of the 32 character hexadecimal result as an int.
     *value = HexStringToInt(hex_md5.substr(24));
     return NPERR_NO_ERROR;
   }
   return NPERR_GENERIC_ERROR;
 }
@@ skipping 960 matching lines @@
   StretchDIBits(dc, printable_area.x(), printable_area.y(),
                 printable_area.width(), printable_area.height(),
                 0, 0, bitmap.width(), bitmap.height(),
                 &compressed_image.front(),
                 reinterpret_cast<const BITMAPINFO*>(&bmi),
                 DIB_RGB_COLORS, SRCCOPY);
   return true;
 }
 #endif  // OS_WIN