Move subpixelTextPositioning flag to FontDescription

The useSubpixelTextPositioning flag can be toggled though TestRunner
(on Linux) and thus needs to be part of the FontDescription.

This change moves the flag to FontDescription so that the cache is
aware of it and return the correct FontData.

BUG=290411
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/75463006

git-svn-id: svn://svn.chromium.org/blink/trunk@162263 bbb929c8-8fbe-4397-9dbb-9b2b20218538