Move Touch to base namespace, remove SetLastModifiedTime.

This implements callers of SetLastModifiedTime in terms of base::Touch.
SetLastModifiedTime was just a shorthand for touch that confusingly set
both the last modified and last access times, and was only used by 7 files.
It seems better to just have the callers call Touch directly.

BUG=
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/100453006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238803 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238803