linux: Enable subpixel positioning correctly in FontPlatformData.

Fix what seems like a bug in the Linux implementation of
FontPlatformData::setupPaint() that resulted in text not
being subpixel positioned when a NULL GraphicsContext was
passed.

BUG=399031

Review URL: https://codereview.chromium.org/431743002

git-svn-id: svn://svn.chromium.org/blink/trunk@179260 bbb929c8-8fbe-4397-9dbb-9b2b20218538