Remove attempt at subpixel handling from GDI font host.

[bungeman, 2012-01-05 16:14:49]

The kSubpixelPositioning_Flag currently means both subpixel advances and glyphs.
The CoreGraphics and FreeType font hosts currently treat this flag this way. The
GDI font host currently will give subpixel advances but not subpixel glyphs.
This change adds kSubpixelPositioning_Flag to the list GDI's ignored flags.

This change should not affect chromium as, while it turns on subpixel
positioning, the Skia GDI font host is not used for metrics.

[reed1, 2012-01-05 16:30:04]

lgtm

[bungeman, 2012-01-05 20:18:43]

Committed revision 2976.