[Predator] Pass flags to callstack detector and fix nit.

appengine/findit/crash/flag_manager.py

Index: appengine/findit/crash/flag_manager.py
diff --git a/appengine/findit/crash/flag_manager.py b/appengine/findit/crash/flag_manager.py
index 8cd3009e5450030c2df4722a70b381062da6cad2..77916ab480566809ad019cd9c3bd78244cb2a0d7 100644
--- a/appengine/findit/crash/flag_manager.py
+++ b/appengine/findit/crash/flag_manager.py
@@ -57,7 +57,7 @@ class ParsingFlag(object):
       value (bool): Initial value of the flag.
     """
     self._name = name
-    self._turn_on_condition = turn_on_condition
+    self._turn_on_condition = turn_on_condition or (lambda _: False)
     self._value = value
 
   @property
@@ -81,8 +81,7 @@ class ParsingFlag(object):
 
   def ConditionallyTurnOn(self, line):
     """When the flag is off, turns on it if turn_on_conditions met."""
-    if (not self._value and self._turn_on_condition and
-        self._turn_on_condition(line)):
+    if not self._value and self._turn_on_condition(line):
       self.TurnOn()