Log serial read and write errors on Windows.

Log platform errors so that it is possible to determine from logs what
caused the chrome.serial API to return the generic "system_error"
message.

Review URL: https://codereview.chromium.org/1147223002

Cr-Commit-Position: refs/heads/master@{#330870}

device/serial/serial_io_handler_win.cc

@@ -211,6 +211,7 @@ void SerialIoHandlerWin::WriteImpl() {
                         NULL,
                         &write_context_->overlapped);
   if (!ok && GetLastError() != ERROR_IO_PENDING) {
+    VPLOG(1) << "Write failed";
     QueueWriteCompleted(0, serial::SEND_ERROR_SYSTEM_ERROR);
   }
 }
@@ -301,6 +302,7 @@ void SerialIoHandlerWin::OnIOCompleted(
                            NULL,
                            &read_context_->overlapped);
       if (!ok && GetLastError() != ERROR_IO_PENDING) {
+        VPLOG(1) << "Read failed";
         ReadCompleted(0, serial::RECEIVE_ERROR_SYSTEM_ERROR);
       }
     }