[Linux] Enable subpixel text on high-dpi regardless of hinting

Expose deviceScaleFactor on GraphicsContext and use to selectively
enable subpixel text regardless of the system font hinting settings for
high-dpi systems.

BUG=373441
R=derat@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/407553002

git-svn-id: svn://svn.chromium.org/blink/trunk@178485 bbb929c8-8fbe-4397-9dbb-9b2b20218538