[DirectWrite] Consider font fallback when determining aliasing

Consider the script resolved during font-fallback when determining the
minimum anti-aliasing font size. This matches the logic in
createFontPlatformData and ensures that CJK characters rendering using a
fallback font are legible.

R=dglazkov@chromium.org
BUG=357864

Review URL: https://codereview.chromium.org/222543002

git-svn-id: svn://svn.chromium.org/blink/trunk@170683 bbb929c8-8fbe-4397-9dbb-9b2b20218538