[DevTools] Fix replace script-based uiSourceCode with network-based

Script-based and network-based source codes can have empty url and in this situtation devtools can replace correct debug-based ui source code with random network-based.

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/684093007

git-svn-id: svn://svn.chromium.org/blink/trunk@184714 bbb929c8-8fbe-4397-9dbb-9b2b20218538