DevTools: do not show "there are unsaved changes" dialog for network resources

DevTools shows "there are unsaved changes" dialog if there's at least one
dirty uiSourceCode. However, saving a network-originated javascript file will
not prevent you from loosing changes as the devtools will close.

This patch changes the behavior so that the alerting message pops up only
if there are file system uiSourceCodes with unsaved changes.

R=vsevik
BUG=426783

Review URL: https://codereview.chromium.org/698053003

git-svn-id: svn://svn.chromium.org/blink/trunk@184856 bbb929c8-8fbe-4397-9dbb-9b2b20218538