DevTools: Make addFileForURL more robust

Eliminate the error:
"Uncaught URIError: URI malformed", source: chrome-devtools://devtools/bundled/sdk/NetworkWorkspaceBinding.js (133)

Review URL: https://codereview.chromium.org/368643002

git-svn-id: svn://svn.chromium.org/blink/trunk@177449 bbb929c8-8fbe-4397-9dbb-9b2b20218538