Change svg/text/alt-glyph-for-surrogate-pair.svg to support subpixel

Update the svg/text/alt-glyph-for-surrogate-pair.svg test to support
subpixel text.

R=pdr@chromium.org

Review URL: https://codereview.chromium.org/351043006

git-svn-id: svn://svn.chromium.org/blink/trunk@176952 bbb929c8-8fbe-4397-9dbb-9b2b20218538