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CopyTreeWorkItem and MoveTreeWorkItem were using the name temp_dir,
while DeleteTreeWorkItem was using the name temp_path.  The latter
is more accurate and this CL makes the three consistent.

Change-Id: I3a9cf5412180e6006fb1afad322b8dde983eb2df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2213360Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Brian Manthos <brianman@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#771431}