Simplify the MediaGalleriesPlatformAppBrowserTest copyTo tests.

Review URL: https://codereview.chromium.org/24356005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225393 0039d316-1c4b-4281-b951-d872f2087c98
Implement preserving last modified time on copy or move.

This CL is the implementation of preserving last modified time on Copy or Move
operations for native file system.

BUG=282107
TEST=Ran content_unittests and unit_tests

Review URL: https://codereview.chromium.org/24368002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225394 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225394