Start action processing asynchronously in UpdateAttempter.

This is done so that we unblock the event loop on dbus calls.

BUG=chromium-os:12758
TEST=unit tests, tested updating the device

Change-Id: I38d9869afb392264a020fc6c653a20622fd38ada

Review URL: http://codereview.chromium.org/6624082
diff --git a/update_attempter.h b/update_attempter.h
index 0afe7f6..e41cab9 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -158,6 +158,9 @@
   static gboolean StaticManagePriorityCallback(gpointer data);
   bool ManagePriorityCallback();
 
+  // Callback to start the action processor.
+  static gboolean StaticStartProcessing(gpointer data);
+
   // Checks if a full update is needed and forces it by updating the Omaha
   // request params.
   void DisableDeltaUpdateIfNeeded();