DevTools: represent ComputedStyle with simple Map

Source/devtools/front_end/elements/StylesSidebarPane.js

Index: Source/devtools/front_end/elements/StylesSidebarPane.js
diff --git a/Source/devtools/front_end/elements/StylesSidebarPane.js b/Source/devtools/front_end/elements/StylesSidebarPane.js
index c43345e06313107d60541757e75db42aad324c0a..7b9a5c10de70f305bc1eb210e9347991336f4855 100644
--- a/Source/devtools/front_end/elements/StylesSidebarPane.js
+++ b/Source/devtools/front_end/elements/StylesSidebarPane.js
@@ -2065,13 +2065,13 @@ WebInspector.StylePropertyTreeElement.prototype = {
         var swatchIcon = new WebInspector.ColorSwatchPopoverIcon(this, stylesPopoverHelper, text);
 
         /**
-         * @param {?WebInspector.CSSStyleDeclaration} styles
+         * @param {?Map.<string, string>} styles
          */
         function computedCallback(styles)
         {
             if (!styles)
                 return;
-            var bgColorText = styles.getPropertyValue("background-color") || "";
+            var bgColorText = styles.get("background-color") || "";
             var bgColor = WebInspector.Color.parse(bgColorText);
             // TODO(aboxhall): for background color with alpha, compute the actual
             // visible background color (blended with content underneath).