Upgrade SVGTextLayoutEngine's dumpTextBoxes() to use String::substring

In debugging crbug.com/264211 I found dumpTextBoxes no longer compiles
with DUMP_TEXT_FRAGMENTS=1 because it uses outdated technology based
on bloated characters. This patch upgrades dumpTextBoxes to compile
with the new String function.

BUG=264211

Review URL: https://chromiumcodereview.appspot.com/21145006

git-svn-id: svn://svn.chromium.org/blink/trunk@155154 bbb929c8-8fbe-4397-9dbb-9b2b20218538