DevTools: Set breakpoints in debugger even if script mapping is not set yet.

R=pfeldman, aandrey
BUG=340318

Review URL: https://codereview.chromium.org/195073002

git-svn-id: svn://svn.chromium.org/blink/trunk@169023 bbb929c8-8fbe-4397-9dbb-9b2b20218538