Use unsigned for GlyphBuffer size and offsets

Change GlyphBuffer and Font classes to use unsigned instead of signed integers for sizes and indices.

Review URL: https://codereview.chromium.org/59873004

git-svn-id: svn://svn.chromium.org/blink/trunk@161413 bbb929c8-8fbe-4397-9dbb-9b2b20218538