Replace use of deprecated CG methods.

OSX10.9 and iOS7.0 deprecated CGContextShowGlyphsAtPoint so a new API
should be used. OSX10.7 and iOS4.2 replace CGContextShowGlyphsAtPoint with
CTFontDrawGlyphs. OSX10.5 and iOS2.0 have CGContextShowGlyphsAtPositions
which works similarly to CTFontDrawGlyphs and has not yet been deprecated.
This change allows the use of CTFontDrawGlyphs when it is available,
falling back to CGContextShowGlyphsAtPositions when it isn't.

Review URL: https://codereview.chromium.org/770383002