[serial] Only add a watcher if if the immeidate read attempt fails.

device/serial/serial_io_handler_posix.h

 // 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.
 
 #ifndef DEVICE_SERIAL_SERIAL_IO_HANDLER_POSIX_H_
 #define DEVICE_SERIAL_SERIAL_IO_HANDLER_POSIX_H_
 
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
@@ skipping 31 matching lines @@
 
  private:
   friend class SerialIoHandler;
   friend class SerialIoHandlerPosixTest;
 
   SerialIoHandlerPosix(
       scoped_refptr<base::SingleThreadTaskRunner> file_thread_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> ui_thread_task_runner);
   ~SerialIoHandlerPosix() override;
 
-  void AttemptRead(bool within_read);
+  bool AttemptRead(bool within_read);
   void RunReadCompleted(bool within_read,
                         int bytes_read,
                         serial::ReceiveError error);
 
   // base::MessageLoopForIO::Watcher implementation.
   void OnFileCanWriteWithoutBlocking(int fd) override;
   void OnFileCanReadWithoutBlocking(int fd) override;
 
   void EnsureWatchingReads();
   void EnsureWatchingWrites();
 
   base::MessageLoopForIO::FileDescriptorWatcher file_read_watcher_;
   base::MessageLoopForIO::FileDescriptorWatcher file_write_watcher_;
 
   // Flags indicating if the message loop is watching the device for IO events.
   bool is_watching_reads_;
   bool is_watching_writes_;
 
   ErrorDetectState error_detect_state_;
   bool parity_check_enabled_;
   char chars_stashed_[2];
   int num_chars_stashed_;
 
   DISALLOW_COPY_AND_ASSIGN(SerialIoHandlerPosix);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_SERIAL_SERIAL_IO_HANDLER_POSIX_H_