Update decomposed-after-stacked-diacritics.html for subpixel

Update the fast/text/decomposed-after-stacked-diacritics.html test to
support subpixel text.

BUG=378610
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/359463002

git-svn-id: svn://svn.chromium.org/blink/trunk@178474 bbb929c8-8fbe-4397-9dbb-9b2b20218538