IndexedDB/Inspector: Avoid dereferencing NULL in render process.

Check output of ScriptValue::toJSONValue before calling toJSONString.

BUG=413377

Review URL: https://codereview.chromium.org/571653004

git-svn-id: svn://svn.chromium.org/blink/trunk@182275 bbb929c8-8fbe-4397-9dbb-9b2b20218538