Don't use getPropertyCSSValue in treeoutline.js

Source/devtools/front_end/ui/treeoutline.js

Index: Source/devtools/front_end/ui/treeoutline.js
diff --git a/Source/devtools/front_end/ui/treeoutline.js b/Source/devtools/front_end/ui/treeoutline.js
index 2ffac7251d691fc0ac2700096902dc339e38dccf..764dde7a7cb2f4f8eb5183d9264159dea8bcb610 100644
--- a/Source/devtools/front_end/ui/treeoutline.js
+++ b/Source/devtools/front_end/ui/treeoutline.js
@@ -1019,8 +1019,9 @@ TreeElement.prototype.traversePreviousTreeElement = function(skipUnrevealed, don
 TreeElement.prototype.isEventWithinDisclosureTriangle = function(event)
 {
     // FIXME: We should not use getComputedStyle(). For that we need to get rid of using ::before for disclosure triangle. (http://webk.it/74446)
-    var paddingLeftValue = window.getComputedStyle(this._listItemNode).getPropertyCSSValue("padding-left");
-    var computedLeftPadding = paddingLeftValue ? paddingLeftValue.getFloatValue(CSSPrimitiveValue.CSS_PX) : 0;
+    var paddingLeftValue = window.getComputedStyle(this._listItemNode).paddingLeft;
+    console.assert(paddingLeftValue.endsWith("px"));
+    var computedLeftPadding = parseFloat(paddingLeftValue, 10);
     var left = this._listItemNode.totalOffsetLeft() + computedLeftPadding;
     return event.pageX >= left && event.pageX <= left + this.arrowToggleWidth && this.hasChildren;
 }