Dispatch geolocation IPCs on the UI thread. Aside from simplifying the code to avoid a lot of threa…

base/callback_list.h

Index: base/callback_list.h
===================================================================
--- base/callback_list.h	(revision 269041)
+++ base/callback_list.h	(working copy)
@@ -89,10 +89,13 @@
     }
 
     ~Subscription() {
-      if (list_->active_iterator_count_)
+      if (list_->active_iterator_count_) {
         iter_->Reset();
-      else
+      } else {
         list_->callbacks_.erase(iter_);
+        if (!list_->removal_callback_.is_null())
+          list_->removal_callback_.Run();
+      }
     }
 
    private:
@@ -111,6 +114,13 @@
         new Subscription(this, callbacks_.insert(callbacks_.end(), cb)));
   }
 
+  // Sets a callback which will be run when a subscription list is changed.
+  void set_removal_callback(const Closure& callback) {
+    removal_callback_ = callback;
+  }
+
+  bool empty() { return callbacks_.empty(); }
+
  protected:
   // An iterator class that can be used to access the list of callbacks.
   class Iterator {
@@ -167,17 +177,24 @@
   // iteration.
   void Compact() {
     typename std::list<CallbackType>::iterator it = callbacks_.begin();
+    bool updated = false;
     while (it != callbacks_.end()) {
-      if ((*it).is_null())
+      if ((*it).is_null()) {
+        updated = true;
         it = callbacks_.erase(it);
-      else
+      } else {
         ++it;
+      }
+
+      if (updated && !removal_callback_.is_null())
+        removal_callback_.Run();
     }
   }
 
  private:
   std::list<CallbackType> callbacks_;
   int active_iterator_count_;
+  Closure removal_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(CallbackListBase);
 };