DevTools: Avoid exception when inspecting blob-based workers

The blob: scheme is not handled by ParsedURL, so it is
impossible to compute the human-readable worker name
for them using the common scheme.

R=pfeldman, vsevik
BUG=404098
NOTRY=true

Review URL: https://codereview.chromium.org/471413002

git-svn-id: svn://svn.chromium.org/blink/trunk@180356 bbb929c8-8fbe-4397-9dbb-9b2b20218538