Remove the unused FragmentationDisabler

Nothing uses this class, lets remove it.

R=eseidel@chromium.org,ojan@chromium.org

Review URL: https://codereview.chromium.org/132803002

git-svn-id: svn://svn.chromium.org/blink/trunk@164829 bbb929c8-8fbe-4397-9dbb-9b2b20218538