Update comment now that I understand it better.

Tools/Scripts/webkitpy/tool/commands/rebaseline.py

 # Copyright (c) 2010 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 430 matching lines @@
         return optimize_commands
 
     def _update_expectations_files(self, lines_to_remove):
         # FIXME: This routine is way too expensive. We're creating O(n ports) TestExpectations objects.
         # This is slow and uses a lot of memory.
         tests = lines_to_remove.keys()
         to_remove = []
 
         # This is so we remove lines for builders that skip this test, e.g. Android skips most
         # tests and we don't want to leave stray [ Android ] lines in TestExpectations..
-        # FIXME: Is this necessary at all now that _rebaseline_commands includes the builders that
-        # used to be skipped because the result wasn't failing on tip of tree?
+        # This is only necessary for "webkit-patch rebaseline" and for rebaselining expected
+        # failures from garden-o-matic. rebaseline-expectations and auto-rebaseline will always
+        # pass the exact set of ports to rebaseline.
         for port_name in self._tool.port_factory.all_port_names():
             port = self._tool.port_factory.get(port_name)
             generic_expectations = TestExpectations(port, tests=tests, include_overrides=False)
             full_expectations = TestExpectations(port, tests=tests, include_overrides=True)
             for test in tests:
                 if self._port_skips_test(port, test, generic_expectations, full_expectations):
                     for test_configuration in port.all_test_configurations():
                         if test_configuration.version == port.test_configuration().version:
                             to_remove.append((test, test_configuration))
 
@@ skipping 466 matching lines @@
                 if options.verbose:
                     rebaseline_command.append('--verbose')
                 # Use call instead of run_command so that stdout doesn't get swallowed.
                 tool.executive.call(rebaseline_command)
             except:
                 traceback.print_exc(file=sys.stderr)
                 # Sometimes git crashes and leaves us on a detached head.
                 tool.scm().checkout_branch(old_branch_name)
 
             time.sleep(self.SLEEP_TIME_IN_SECONDS)