SVGTextMetrics shouldn't use bloatedCharacters

Instead of manually re-implementing substring, we should just call substring.

R=pdr@chromium.org
BUG=254708

Review URL: https://codereview.chromium.org/19317003

git-svn-id: svn://svn.chromium.org/blink/trunk@154272 bbb929c8-8fbe-4397-9dbb-9b2b20218538