Move shouldSmoothFonts out of GraphicsContextState.

The flag is an invariant GraphicsContext property (only set once, at
construction time) and doesn't need to be saved/restored as part of
GraphicsContextState.

R=schenney@chromium.org,senorblanco@chromium.org,jbroman@chromium.org

Review URL: https://codereview.chromium.org/494773003

git-svn-id: svn://svn.chromium.org/blink/trunk@180735 bbb929c8-8fbe-4397-9dbb-9b2b20218538