DevTools: simplify WI.CSSProperty and WI.CSSStyleDeclaration interfaces

LayoutTests/inspector/elements/styles-2/inject-stylesheet-expected.txt

 CONSOLE MESSAGE: line 10: iframe loaded
 Tests that injected user stylesheets are reflected in the Styles pane.
 
 
 Main frame style:
 background-attachment: scroll;
 background-clip: border-box;
 background-color: rgb(0, 0, 255);
 background-image: none;
 background-origin: padding-box;
@@ skipping 53 matching lines @@
         background-repeat-x: initial;
         background-repeat-y: initial;
         background-attachment: initial;
         background-origin: initial;
         background-clip: initial;
         background-color: red;
 
 [expanded] 
 media="screen" (inject-stylesheet-iframe-data.html)
 body  { (inject-styleshe…me-data.html:4 -> inject-stylesheet-iframe-data.html:4:3)
-    background: green;
+/-- overloaded --/     background: green;
     /-- overloaded --/     background-image: initial;
     /-- overloaded --/     background-position-x: initial;
     /-- overloaded --/     background-position-y: initial;
     /-- overloaded --/     background-size: initial;
     /-- overloaded --/     background-repeat-x: initial;
     /-- overloaded --/     background-repeat-y: initial;
     /-- overloaded --/     background-attachment: initial;
     /-- overloaded --/     background-origin: initial;
     /-- overloaded --/     background-clip: initial;
     /-- overloaded --/     background-color: green;
 
 [expanded] 
 body  { (user agent stylesheet)
     display: block;
     margin: 8px;
         margin-top: 8px;
         margin-right: 8px;
         margin-bottom: 8px;
         margin-left: 8px;