Revert 109528 - Use XFixes to update the clipboard sequence number.

      BUG=73478
TEST=manual testing

Review URL: http://codereview.chromium.org/8501002

TBR=dcheng@chromium.org
Review URL: http://codereview.chromium.org/8524014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@109530 0039d316-1c4b-4281-b951-d872f2087c98