Fix experimental sub-pixel font scaling for webfonts on windows

The experimental sub-pixel scaling support on windows was broken for
webfonts by one of the many recent font refactorings.

This change reintroduces the functionality by having the
FontPlatformData constructors useSubpixelPositioning argument default
to the system default, as set by the font cache.

BUG=290411
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/111283002

git-svn-id: svn://svn.chromium.org/blink/trunk@163574 bbb929c8-8fbe-4397-9dbb-9b2b20218538