Fix Wombat in Safari 9

In Safari 9, Object.getOwnPropertyDescriptor(domObject.prototype, prop)
returns descriptors which have the correct structure but undefined
getters and are marked as unconfigurable.

See https://bugs.webkit.org/show_bug.cgi?id=49739#c19
for details. The getters cannot be retrieved via obj.__lookupGetter__()
either.

 * Resolve the issue by skipping overrides for DOM properties
   where the property is not configurable, or the original getter for
   a property could not be retrieved.

 * Lookup the 'baseURI' property on the correct prototype (Node,
   not document)

This fix also resolves the problem where accesses to document.baseURI
on Edge would fail.

Author: robertknight

pywb/static/wombat.js

@@ -363,26 +363,31 @@ var wombat_internal = function($wbwindow) {
     }
 
     //============================================
-    // Define custom property
+    // Override a DOM property
     function def_prop(obj, prop, set_func, get_func) {
+        // if the property is marked as non-configurable in the current
+        // browser, skip the override
+        var existingDescriptor = Object.getOwnPropertyDescriptor(obj, prop);
+        if (existingDescriptor && !existingDescriptor.configurable) {
+            return;
+        }
+
+        // if no getter function was supplied, skip the override.
+        // See https://github.com/ikreymer/pywb/issues/147 for context
+        if (!get_func) {
+            return;
+        }
+
         try {
             Object.defineProperty(obj, prop, {
                 configurable: false,
-//                enumerable: true,
                 set: set_func,
                 get: get_func
             });
 
             return true;
         } catch (e) {
-            var info = "Can't redefine prop " + prop;
-            console.warn(info);
-            //f (obj && obj.tagName) {
-            //    info += " on " + obj.tagName;
-            //}
-            //if (value != obj[prop]) {
-            //    obj[prop] = value;
-            //}
+            console.warn('Failed to redefine property %s', prop, e.message);
             return false;
         }
     }
@@ -757,15 +762,16 @@ var wombat_internal = function($wbwindow) {
         def_prop($wbwindow.HTMLBaseElement.prototype, "href", undefined, base_href_get);
 
         // Shared baseURI
-        var orig_getter = $wbwindow.document.__lookupGetter__("baseURI");
+        var orig_getter = get_orig_getter($wbwindow.Node, "baseURI");
+        if (orig_getter) {
+            var get_baseURI = function() {
+                var res = orig_getter.call(this);
+                return extract_orig(res);
+            }
 
-        var get_baseURI = function() {
-            var res = orig_getter.call(this);
-            return extract_orig(res);
+            def_prop($wbwindow.HTMLElement.prototype, "baseURI", undefined, get_baseURI);
+            def_prop($wbwindow.HTMLDocument.prototype, "baseURI", undefined, get_baseURI);
         }
-
-        def_prop($wbwindow.HTMLElement.prototype, "baseURI", undefined, get_baseURI);
-        def_prop($wbwindow.HTMLDocument.prototype, "baseURI", undefined, get_baseURI);
     }
 
     //============================================