Teach SVG about 8 bit TextRuns

This CL teach SVG how to handle 8 bit text runs, which removes
a number of callers of bloatedCharacters().

R=eseidel@chromium.org, pdr@chromium.org
BUG=254708

Review URL: https://codereview.chromium.org/19235004

git-svn-id: svn://svn.chromium.org/blink/trunk@154274 bbb929c8-8fbe-4397-9dbb-9b2b20218538