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

For some USB drives, writes on Mac are much slower than using the old image
creator. This is most likely because the implementation in Chrome uses
/dev/disk# whereas the old implementation used /dev/rdisk# whenever
possible.

The main requirement is that writes are in multiples of the block size, but
since ImageWriter::WriteChunk always writes in multiples of
kMemoryAlignment (4096), it should be fine to always use the raw disk.

My brief tests using a Lexar JumpDrive 16GB and a 1.6GB image:
before: ~6.5 minutes to write, same to verify
after: ~5 minutes to write, ~1.5 minutes to verify

BUG=415891

Review URL: https://codereview.chromium.org/599853002

Cr-Commit-Position: refs/heads/master@{#296875}

chrome/utility/image_writer/image_writer.cc

@@ -125,6 +125,7 @@ void ImageWriter::WriteChunk() {
     // 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);
 

chrome/utility/image_writer/image_writer_mac.cc

@@ -95,9 +95,15 @@ bool ImageWriter::OpenDevice() {
   // Find the file path to open.
   base::FilePath real_device_path;
   if (device_path_.IsAbsolute()) {
+    // This only occurs for tests where the device path is mocked with a
+    // temporary file.
     real_device_path = device_path_;
   } else {
-    real_device_path = base::FilePath("/dev").Append(device_path_);
+    // Get the raw device file. Writes need to be in multiples of
+    // DAMediaBlockSize (usually 512). This is fine since WriteChunk() writes in
+    // multiples of kMemoryAlignment.
+    real_device_path =
+        base::FilePath("/dev").Append("r" + device_path_.BaseName().value());
   }
 
   // Build the command line.