Bug 609592 - Win7 JumpList is freeing up resources too early for History.

r=jimm a=gavin

browser/components/wintaskbar/WindowsJumpLists.jsm

@@ -260,10 +260,7 @@ var WinTaskbarJumpList =
 
   _pendingStatements: {},
   _hasPendingStatements: function WTBJL__hasPendingStatements() {
-    for (let listType in this._pendingStatements) {
-      return true;
-    }
-    return false;
+    return Object.keys(this._pendingStatements).length > 0;
   },
 
   _buildList: function WTBJL__buildList() {
@@ -344,6 +341,11 @@ var WinTaskbarJumpList =
   },
 
   _buildFrequent: function WTBJL__buildFrequent() {
+    // If history is empty, just bail out.
+    if (!PlacesUtils.history.hasHistoryEntries) {
+      return;
+    }
+
     // Windows supports default frequent and recent lists,
     // but those depend on internal windows visit tracking
     // which we don't populate. So we build our own custom
@@ -377,6 +379,11 @@ var WinTaskbarJumpList =
   },
 
   _buildRecent: function WTBJL__buildRecent() {
+    // If history is empty, just bail out.
+    if (!PlacesUtils.history.hasHistoryEntries) {
+      return;
+    }
+
     var items = Cc["@mozilla.org/array;1"].
                 createInstance(Ci.nsIMutableArray);
     // Frequent items will be skipped, so we select a double amount of