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This creates a basic FileOperations interface for use with file
transfers that should be relatively straightforward to proxy over IPC.

It also creates a ThreadedFileOperations implementation of the
FileOperations interface using FileProxy, which will eventually replace
FileProxyWrapper. This implementation should be platform agnostic. A
future CL will add an IpcFileOperations implementation for Windows, which
will proxy operations from the network process to a ThreadedFileOperation
instance in the desktop process.

Bug: 679313
Change-Id: Ic7d6ce35f498e469f86f9ed7a8fbcc8bd0cafda9
Reviewed-on: https://chromium-review.googlesource.com/c/1325213Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612377}