DevTools: [CSS] promisify CSS domain

LayoutTests/inspector/elements/styles-4/styles-source-offsets.html

Index: LayoutTests/inspector/elements/styles-4/styles-source-offsets.html
diff --git a/LayoutTests/inspector/elements/styles-4/styles-source-offsets.html b/LayoutTests/inspector/elements/styles-4/styles-source-offsets.html
index 1b47cac26e902d5f55b930533656fbaea00019e3..9cfc402e4ad58902b4615166153336682eb877cf 100644
--- a/LayoutTests/inspector/elements/styles-4/styles-source-offsets.html
+++ b/LayoutTests/inspector/elements/styles-4/styles-source-offsets.html
@@ -61,13 +61,15 @@ function test()
              return;
         }
         resultStyles.matchedCSSRules = matchedCSSRules;
-        step2();
     }
 
     function step1(node)
     {
-        InspectorTest.CSSAgent.getInlineStylesForNode(node.id, inlineCallback);
-        InspectorTest.CSSAgent.getMatchedStylesForNode(node.id, matchedCallback);
+        var promises = [
+            InspectorTest.CSSAgent.getInlineStylesForNode(node.id, inlineCallback),
+            InspectorTest.CSSAgent.getMatchedStylesForNode(node.id, matchedCallback)
+        ];
+        Promise.all(promises).then(step2);
     }
 
     function step2()