DevTools: Get rid of isEditable flag on UISourceCode

Clients could decide whether they need to provide edit support based on the uiSourceCode origin.

R=pfeldman

Review URL: https://codereview.chromium.org/236203022

git-svn-id: svn://svn.chromium.org/blink/trunk@172356 bbb929c8-8fbe-4397-9dbb-9b2b20218538