[Devtools] Lazily build reverse mappings

third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js

Index: third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
diff --git a/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js b/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
index 0a10c72cb5445a7852105235bdf23cceb0f928ca..40e7bb6e5f29264b80150dac7a6aedd7c1ff9194 100644
--- a/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
+++ b/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
@@ -212,7 +212,9 @@ WebInspector.CSSModel.prototype = {
      */
     _factoryForSourceMap: function(sourceMap)
     {
-        var sourceExtensions = new Set(sourceMap.sourceURLs().map(url => WebInspector.ParsedURL.extractExtension(url)));
+        var sourceExtensions = new Set();
+        for (const url of sourceMap.sourceURLs())
+            sourceExtensions.add(WebInspector.ParsedURL.extractExtension(url));
         for (var runtimeExtension of self.runtime.extensions(WebInspector.SourceMapFactory)) {
             var supportedExtensions = new Set(runtimeExtension.descriptor()["extensions"]);
             if (supportedExtensions.containsAll(sourceExtensions))