Full JSON file has to be read instead of first line.

In GenerateDashBoard class only the first line of json files is read.
It will fail if we have multi-line JSON files.

Review URL: https://codereview.chromium.org/576003002

git-svn-id: svn://svn.chromium.org/blink/trunk@182168 bbb929c8-8fbe-4397-9dbb-9b2b20218538