DevTools: [Workspace] fix removing stylesheet-related uiSourceCodes

The patch fixes a bug in removing uiSourceCodes from workspace as
the associated stylesheet gets removed.

BUG=424544
R=vsevik

Review URL: https://codereview.chromium.org/661923005

git-svn-id: svn://svn.chromium.org/blink/trunk@183915 bbb929c8-8fbe-4397-9dbb-9b2b20218538