DevTools: Add a checkbox to enable async stacks in the sidebar header.

Source/devtools/front_end/DebuggerModel.js

Index: Source/devtools/front_end/DebuggerModel.js
diff --git a/Source/devtools/front_end/DebuggerModel.js b/Source/devtools/front_end/DebuggerModel.js
index b772d03acb2366a455275bd10d2a99351eab92e0..721d8ea942a6170afc0c88effc99b0190f10263c 100644
--- a/Source/devtools/front_end/DebuggerModel.js
+++ b/Source/devtools/front_end/DebuggerModel.js
@@ -50,7 +50,6 @@ WebInspector.DebuggerModel = function()
     WebInspector.settings.pauseOnExceptionStateString.addChangeListener(this._pauseOnExceptionStateChanged, this);
 
     WebInspector.settings.enableAsyncStackTraces.addChangeListener(this._asyncStackTracesStateChanged, this);
-    WebInspector.settings.maxAsyncStackTraceDepth.addChangeListener(this._asyncStackTracesStateChanged, this);
 
     this.enableDebugger();
 
@@ -180,9 +179,9 @@ WebInspector.DebuggerModel.prototype = {
 
     _asyncStackTracesStateChanged: function()
     {
+        const maxAsyncStackChainDepth = 4;
         var enabled = WebInspector.settings.enableAsyncStackTraces.get();
-        var depth = enabled ? Number(WebInspector.settings.maxAsyncStackTraceDepth.get()) : 0;
-        DebuggerAgent.setAsyncCallStackDepth(depth);
+        DebuggerAgent.setAsyncCallStackDepth(enabled ? maxAsyncStackChainDepth : 0);
     },
 
     _debuggerWasDisabled: function()