check-webkit-style failing with "Path does not exist."

check-webkit-style failing with "Path does not exist."
https://bugs.webkit.org/show_bug.cgi?id=77873

Reviewed by Ojan Vafai.

This change fixes the way the style checker determines which
Port class to use for a given test_expectations.txt path; the
previous version used a heuristic that didn't really work in the
first place.

* Scripts/webkitpy/style/checkers/test_expectations.py:
(TestExpectationsChecker._determine_port_from_expectations_path):
(TestExpectationsChecker.__init__):
* Scripts/webkitpy/style/checkers/test_expectations_unittest.py:
(TestExpectationsTestCase._expect_port_for_expectations_path):
(TestExpectationsTestCase.test_determine_port_from_expectations_path):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107124 268f45cc-cd09-0410-ab3c-d52691b4dbfc