Add GetMainDom method to UmbracoApplication so GetRuntime can be overridden

(cherry picked from commit c3f59602c2)

src/Umbraco.Web/UmbracoApplication.cs

@@ -2,6 +2,7 @@
 using System.Threading;
 using System.Web;
 using Umbraco.Core;
+using Umbraco.Core.Logging;
 using Umbraco.Core.Logging.Serilog;
 using Umbraco.Core.Runtime;
 using Umbraco.Web.Runtime;
@@ -17,16 +18,24 @@ namespace Umbraco.Web
         {
             var logger = SerilogLogger.CreateWithDefaultConfiguration();
 
+            var runtime = new WebRuntime(this, logger, GetMainDom(logger));
+
+            return runtime;
+        }
+
+        /// <summary>
+        /// Returns a new MainDom
+        /// </summary>
+        protected IMainDom GetMainDom(ILogger logger)
+        {
             // Determine if we should use the sql main dom or the default
             var appSettingMainDomLock = ConfigurationManager.AppSettings[Constants.AppSettings.MainDomLock];
 
             var mainDomLock = appSettingMainDomLock == "SqlMainDomLock"
                 ? (IMainDomLock)new SqlMainDomLock(logger)
                 : new MainDomSemaphoreLock(logger);
-            
-            var runtime = new WebRuntime(this, logger, new MainDom(logger, mainDomLock));
 
-            return runtime;
+            return new MainDom(logger, mainDomLock);
         }
 
         /// <summary>