Update gdb printing for WTF::Strings after r155464 and r155576.

R=ojan@chromium.org,abarth@chromium.org
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/22661004

git-svn-id: svn://svn.chromium.org/blink/trunk@155908 bbb929c8-8fbe-4397-9dbb-9b2b20218538