Work around CG assert drawing tiny glyphs.

For some fonts when drawing glyphs at CGFLOAT_MIN size,
CG will assert internally. Instead, request such glyphs at size 1,
and scale them down to 0 with a transform.

This change follows the pattern set by
3490263287e9432119c501884e45840de4d986bc
"Replace use of deprecated CG methods." on master.

BUG=chromium:441538
R=reed@google.com

Review URL: https://codereview.chromium.org/796783003