Cosmetic change for disk flush interval in Files.app file copying.

Previously, we have issued a big single flush at the end of the copy
operations. However, the progressbar UI corresponding to it for large
(> 100M) files is "quickly progress to 99%, and keep the user waiting
for dozens of seconds".) This is surely not cool.

This CL changes the behavior to run flush every 10MB progress, which
makes the progress bar to extend more uniformly.

BUG=276276
R=kinuko@chromium.org

Review URL: https://codereview.chromium.org/108323004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241128 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241128