FileEntry.file is slow operation internally queries file metadata.
However, FileTransferController calls FileEntry.file for all the
selected files each time CHANGE_THROTTLED event is fired.
CHANGE_THROTTLED is fired every time a file is copied
during multi file copy.
In some slow storage devices which do not support parallel requests
such as MTP connected ones, this severely slows down file copy.

BUG=712121
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2843683002
Cr-Commit-Position: refs/heads/master@{#467270}