Migrate to FileOperations and remove FileProxyWrapper

This updates FileTransferMessageHandler to use the new FileOperations
interface. Windows is left unimplemented for now, as thas will require
the forthcoming IpcFileOperations implementation.

Bug: 679313
Change-Id: Ia6c31ed4ae5fbc4c2fda4d93cdecbdf11573d3fa
Reviewed-on: https://chromium-review.googlesource.com/c/1357385
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615311}
28 files changed