Bug 923331 - Use more general getter for docshell in browser.xml (r=gavin)

toolkit/content/widgets/browser.xml

@@ -216,9 +216,18 @@
 
       <field name="_docShell">null</field>
 
-      <property name="docShell"
-                onget="return this._docShell || (this._docShell = this.boxObject.QueryInterface(Components.interfaces.nsIContainerBoxObject).docShell);"
-                readonly="true"/>
+      <property name="docShell" readonly="true">
+        <getter><![CDATA[
+          if (this._docShell)
+            return this._docShell;
+
+          let frameLoader = this.QueryInterface(Components.interfaces.nsIFrameLoaderOwner).frameLoader;
+          if (!frameLoader)
+            return null;
+          this._docShell = frameLoader.docShell;
+          return this._docShell;
+        ]]></getter>
+      </property>
 
       <property name="docShellIsActive">
         <getter>