Minor optimization for dynamic type creation in .NET PageFactory

The PageFactory creates a dynamic type which implements the WebDriver
interfaces as a means of having a concrete System.Type to proxy. This
commit moves that dynamic type creation to a single act, so that
additional, unnecssary dynamic types are not created, reducing bloat and
memory footprint.

dotnet/src/support/PageObjects/DefaultPageObjectMemberDecorator.cs

@@ -32,6 +32,7 @@ namespace OpenQA.Selenium.Support.PageObjects
     public class DefaultPageObjectMemberDecorator : IPageObjectMemberDecorator
     {
         private static List<Type> interfacesToBeProxied;
+        private static Type interfaceProxyType;
 
         private static List<Type> InterfacesToBeProxied
         {
@@ -49,6 +50,19 @@ private static List<Type> InterfacesToBeProxied
             }
         }
 
+        private static Type InterfaceProxyType
+        {
+            get
+            {
+                if (interfaceProxyType == null)
+                {
+                    interfaceProxyType = CreateTypeForASingleElement();
+                }
+
+                return interfaceProxyType;
+            }
+        }
+
         /// <summary>
         /// Locates an element or list of elements for a Page Object member, and returns a
         /// proxy object for the element or list of elements.
@@ -184,7 +198,7 @@ private static object CreateProxyObject(Type memberType, IElementLocator locator
             }
             else if (memberType == typeof(IWebElement))
             {
-                proxyObject = WebElementProxy.CreateProxy(CreateTypeForASingleElement(), locator, bys, cache);
+                proxyObject = WebElementProxy.CreateProxy(InterfaceProxyType, locator, bys, cache);
             }
             else
             {