Remove unused GlyphBuffer methods.

  * clear() doesn't have any users
  * reverse() is never used for GlyphBufferWithOffsets

R=eae@chromium.org,rottsches@intel.com,jbroman@chromium.org

Review URL: https://codereview.chromium.org/615263003

git-svn-id: svn://svn.chromium.org/blink/trunk@183083 bbb929c8-8fbe-4397-9dbb-9b2b20218538