[Mac] Use raw disks (/dev/rdisk#) in image_writer_mac.

chrome/utility/image_writer/image_writer.cc

 // 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 "chrome/utility/image_writer/image_writer.h"
 
 #include "base/memory/aligned_memory.h"
 #include "chrome/utility/image_writer/error_messages.h"
 #include "chrome/utility/image_writer/image_writer_handler.h"
 #include "content/public/utility/utility_thread.h"
@@ skipping 107 matching lines @@
   memset(buffer.get(), 0, kBurningBlockSize);
 
   int bytes_read = image_file_.Read(bytes_processed_, buffer.get(),
                                     kBurningBlockSize);
 
   if (bytes_read > 0) {
     // Always attempt to write a whole block, as writing DASD requires sector-
     // aligned writes to devices.
     int bytes_to_write = bytes_read + (kMemoryAlignment - 1) -
                          (bytes_read - 1) % kMemoryAlignment;
+    DCHECK_EQ(0, bytes_to_write % kMemoryAlignment);
     int bytes_written =
         device_file_.Write(bytes_processed_, buffer.get(), bytes_to_write);
 
     if (bytes_written < bytes_read) {
       Error(error::kWriteImage);
       return;
     }
 
     bytes_processed_ += bytes_read;
     PostProgress(bytes_processed_);
@@ skipping 50 matching lines @@
     running_ = false;
   } else {
     // Unable to read entire file.
     LOG(ERROR) << "Failed to read " << kBurningBlockSize << " bytes of image "
                << "at offset " << bytes_processed_;
     Error(error::kReadImage);
   }
 }
 
 }  // namespace image_writer