DevTools: [SSP] refactor internal representations of sections

LayoutTests/inspector/elements/styles/inject-stylesheet-expected.txt

 CONSOLE MESSAGE: line 9: iframe loaded
 Tests that injected user stylesheets are reflected in the Styles pane.
 
 
 Main frame style:
 -webkit-background-clip: border-box;
 -webkit-background-origin: padding-box;
 -webkit-background-size: auto;
 -webkit-border-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAQAAAC1+jfqAAAAiElEQ…Ms+LS30CAhBN5nNxeT5hbJ1zwmji2k+aF6NENIPf/hs54f0sZFUVAMigAAAABJRU5ErkJggg==) 100% / 1 / 0px stretch;
 background-attachment: scroll;
@@ skipping 24 matching lines @@
 
 [expanded] 
 #main  { (injected stylesheet)
   color: red;
   -webkit-border-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAQAAAC1+jfqAAAAiElEQ…Ms+LS30CAhBN5nNxeT5hbJ1zwmji2k+aF6NENIPf/hs54f0sZFUVAMigAAAABJRU5ErkJggg==);
 
 [expanded] 
 div  { (user agent stylesheet)
   display: block;
 
-
 iframe style:
 -webkit-background-clip: border-box;
 -webkit-background-origin: padding-box;
 -webkit-background-size: auto;
 background-attachment: scroll;
 background-clip: border-box;
 background-color: rgb(255, 0, 0);
 background-image: none;
 background-origin: padding-box;
 background-size: auto;
@@ skipping 37 matching lines @@
 [expanded] 
 body  { (user agent stylesheet)
   display: block;
   margin: 8px;
       margin-top: 8px;
       margin-right: 8px;
       margin-bottom: 8px;
       margin-left: 8px;
 
 
-