[serial] Use base::FilePath in device.mojom.SerialPortInfo

This changes the type of the |path| field in device.mojom.SerialPortInfo
from a string to a mojo_base.mojom.FilePath and updates the
SerialDeviceEnumerator and SerialIoHandler implementations to pass
around base::FilePaths instead of std::strings.

This was done in order to make sure that device paths are stored in the
correct platform-specific encoding and can be displayed to the user
using base::FilePath::LossyDisplayName().

Bug: 908836
Change-Id: I46d0d18413c00db87f8ed3e58d73d5ae39b4c51e
Reviewed-on: https://chromium-review.googlesource.com/c/1406087
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621895}
24 files changed