fileManagerPrivate.getFileTasks is slow.
However, every time a file is copied during multi file copy, the method
was called via CHANGE_THROTTLED event listener.
This CL stops creating FileTasks again when file entries for the
previous FileTasks are same as the current ones.
Other possible solution to this problem is to change C++ interface of
fileManagerPrivate.getFileTasks to support incremental update of
FileTasks.

BUG=462989,712121
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2833413003
Cr-Commit-Position: refs/heads/master@{#467264}