DevTools: [SSP] refactor internal representations of sections

LayoutTests/http/tests/inspector/elements/styles/edit-css-with-source-url-expected.txt

 Tests file system project mappings.
 
 
 Running: testEditingRulesInElementsPanelDoesNotAddSourceURLToOriginalFile
 Adding file system.
 Adding file system mapping.
 Dumping uiSourceCode content:
 #inspected {
     color: red;
 }
 
 Loading stylesheet with sourceURL:
 Stylesheet loaded.
 Dumping matched rules:
 [expanded] 
 element.style  { ()
 
 [expanded] 
 #inspected  { (foo.css:1 -> foo.css:1:1)
   color: red;
 
 [expanded] 
 div  { (user agent stylesheet)
   display: block;
 
-
 Editing styles from elements panel:
 Styles edited.
 Dumping matched rules:
 [expanded] 
 element.style  { ()
 
 [expanded] 
 #inspected  { (foo.css:1 -> foo.css:1:1)
   color: green;
   : ;
 
 [expanded] 
 div  { (user agent stylesheet)
   display: block;
 
-
 Dumping uiSourceCode content:
 #inspected {
     color: green;
 }
 
 All done.