Reland with a virtual destructor.

      Use XFixes to update the clipboard sequence number.

BUG=73478
TEST=manual testing

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=109528

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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@109537 0039d316-1c4b-4281-b951-d872f2087c98