DevTools: make extension server a part of core, panels' code should depend on it.

Source/devtools/front_end/main/module.json

 {
     "extensions": [
         {
             "type": "@WebInspector.ContextMenu.Provider",
             "contextTypes": ["WebInspector.UISourceCode", "WebInspector.Resource", "WebInspector.NetworkRequest", "Node"],
             "className": "WebInspector.HandlerRegistry.ContextMenuProvider"
         },
         {
             "type": "@WebInspector.ActionDelegate",
             "actionId": "main.reload",
@@ skipping 208 matching lines @@
             "settingType": "checkbox"
         },
         {
             "type": "ui-setting",
             "section": "Extensions",
             "settingType": "custom",
             "className": "WebInspector.HandlerRegistry.OpenAnchorLocationSettingDelegate"
         }
     ],
     "dependencies": [
+        "extensions",
         "host",
         "screencast",
         "sdk",
         "toolbox"
     ],
     "scripts": [
         "HelpScreenUntilReload.js",
         "App.js",
         "AdvancedApp.js",
         "OverridesView.js",
         "RenderingOptions.js",
         "ScreencastApp.js",
         "SimpleApp.js",
         "TestController.js",
         "Tests.js",
         "Main.js"
     ],
     "skip_compilation": [
         "Tests.js"
     ]
 }