Implement preserving last modified on Drive File System.

This CL implements preserving last modified on Copy()/Move() operation on
Drive File System. The feature is not yet used on production, but will be
used by following CLs.

BUG=282107
TEST=Ran unit_tests
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/24245004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225161 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225161