DevTools: NetworkPanel: use optimistic JSON parser for response preview.

There are some legal tricks used to encode JSON. But JSON.parse rejects
to parse it, whereas XHR response is correctly transformed to JSON.

BUG=406900

Review URL: https://codereview.chromium.org/654083006

git-svn-id: svn://svn.chromium.org/blink/trunk@183821 bbb929c8-8fbe-4397-9dbb-9b2b20218538