DevTools: [Workspace] remove UISourceCode when corresponding CSSStyleSheet gets removed

As of today, stylesheets from CSSStyleModel are added to the workspace,
but never deleted.

The patch teaches NetworkUISourceCodeProvider to listen to
CSSStyleSheet removed events.

R=vsevik

Review URL: https://codereview.chromium.org/615073004

git-svn-id: svn://svn.chromium.org/blink/trunk@183193 bbb929c8-8fbe-4397-9dbb-9b2b20218538