Adds USB writing for OS X.

chrome/utility/image_writer/image_writer_stub.cc

Index: chrome/utility/image_writer/image_writer_stub.cc
diff --git a/chrome/utility/image_writer/image_writer_stub.cc b/chrome/utility/image_writer/image_writer_stub.cc
index 264de9d54c2b9ec758605d09a5c313475e9aee5f..738bd97039c00834c796fe9c5c9b580edaf3406a 100644
--- a/chrome/utility/image_writer/image_writer_stub.cc
+++ b/chrome/utility/image_writer/image_writer_stub.cc
@@ -14,9 +14,16 @@ bool ImageWriter::IsValidDevice() {
   return false;
 }
 
-bool ImageWriter::UnmountVolumes() {
+void ImageWriter::UnmountVolumes(const base::Closure& continuation) {
   NOTIMPLEMENTED();
-  return false;
+}
+
+bool ImageWriter::OpenDevice() {
+  device_file_.Initialize(
+      device_path_,
+      base::File::FLAG_OPEN | base::File::FLAG_READ | base::File::FLAG_WRITE |
+          base::File::FLAG_EXCLUSIVE_READ | base::File::FLAG_EXCLUSIVE_WRITE);
+  return device_file_.IsValid();
 }
 
 }  // namespace image_writer