DevTools: follow up to r166401, added 'new' to call String constructor, not cast.

TBR=apavlov

Review URL: https://codereview.chromium.org/153063004

git-svn-id: svn://svn.chromium.org/blink/trunk@166403 bbb929c8-8fbe-4397-9dbb-9b2b20218538