Consolidate Glyph and GlyphBufferGlyph.

These are always equivalent since the WinCE case was dropped over a year ago:
https://chromiumcodereview.appspot.com/13901012

BUG=383594

Review URL: https://codereview.chromium.org/325283002

git-svn-id: svn://svn.chromium.org/blink/trunk@176039 bbb929c8-8fbe-4397-9dbb-9b2b20218538