This PR fixes the issue mentioned in #83590 for GCE-PD. It uses
WriteVolumeCache API to writes the file system cache to disk during
UnmountDevice in Windows. Linux does not need to explicitly flush cache
because unmount will automatically sync the disk which also flush the
cache.
Change-Id: Ife2745c92b8c0446e79a52e9f9ec7851d2f6b90d