Remove font subpixel positioning flag

Remove the static FontCache::s_useSubpixelPositioning field and the bool
FontPlatformData::m_useSubpixelPositioning field. They are only ever set
explicitly on Windows and always to true since the GDI path was removed.

BUG=579678
R=kojii@chromium.org

Review-Url: https://codereview.chromium.org/1930503003
Cr-Commit-Position: refs/heads/master@{#390337}
9 files changed