DevTools: Strip sourceURL comment from stylesheets before showing them in the editor.

This provides better UX when editing resources that are not mapped in workspace.

R=apavlov, pfeldman_ooo

Review URL: https://codereview.chromium.org/110223008

git-svn-id: svn://svn.chromium.org/blink/trunk@164300 bbb929c8-8fbe-4397-9dbb-9b2b20218538