[Devtools] Lazily build reverse mappings

third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js

Index: third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js
diff --git a/third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js b/third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js
index 5f20ec60fd80ac254bd3bb9442d2d8dc2e2580b7..5d08c9cadab1937ba6ea316ff61b9e1558223028 100644
--- a/third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js
+++ b/third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js
@@ -151,14 +151,14 @@ WebInspector.BlackboxManager.prototype = {
         if (!previousScriptState)
             return Promise.resolve();
 
-        var mappings = sourceMap.mappings().slice();
-        mappings.sort(mappingComparator);
-
+        var hasBlackboxedMappings = sourceMap.sourceURLs().some((url) => this.isBlackboxedURL(url));
+        var mappings = hasBlackboxedMappings ? sourceMap.mappings().slice() : [];
         if (!mappings.length) {
             if (previousScriptState.length > 0)
                 return this._setScriptState(script, []);
             return Promise.resolve();
         }
+        mappings.sort(mappingComparator);
 
         var currentBlackboxed = false;
         var isBlackboxed = false;