The original implementation was syncing file contents to disk
when only a flush was required.

BUG=222623

Review URL: https://codereview.chromium.org/113373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247279 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247279