Fixes <http://webkit.org/b/72635> NRWT crashes when parsing leaks files from which certain
leaks were excluded

Reviewed by Adam Barth.

* Scripts/webkitpy/layout_tests/port/leakdetector.py:
(LeakDetector._parse_leaks_output): Extract just the number of excluded leaks from the
regular expression match, not the entire matched string.

* Scripts/webkitpy/layout_tests/port/leakdetector_unittest.py:
(LeakDetectorTest): Added some example output that shows how things look when leaks are
excluded.
(LeakDetectorTest.test_parse_leaks_output): Test that we parse the number of exclusions
correctly.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@100655 268f45cc-cd09-0410-ab3c-d52691b4dbfc