Fix the PRESUBMIT.py script to properly invoke grit

The grd reader is invoked from _GetGrdMessages with defines which
specify "chromium" build.

Drive-by fix for the MockFile to support basename on Windows.

BUG=984910
TEST=A modified grd fail doesn't trip the presubmit run.

Change-Id: I1b30dcaf8b91d40292e631ba854b4f0dfa54de69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1705799Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678407}

PRESUBMIT.py

@@ -4267,7 +4267,7 @@ def _CheckTranslationScreenshots(input_api, output_api):
     """
     doc = grit.grd_reader.Parse(grd_path_or_string, dir_path,
         stop_after=None, first_ids_file=None,
-        debug=False, defines=None,
+        debug=False, defines={'_chromium': 1},
         tags_to_ignore=set([PART_FILE_TAG]))
     return {
       msg.attrs['name']:msg for msg in doc.GetChildrenOfType(

PRESUBMIT_test_mocks.py

@@ -220,6 +220,10 @@ class MockFile(object):
     """os.path.basename is called on MockFile so we need a len method."""
     return len(self._local_path)
 
+  def replace(self, altsep, sep):
+    """os.path.basename is called on MockFile so we need a replace method."""
+    return self._local_path.replace(altsep, sep)
+
 
 class MockAffectedFile(MockFile):
   def AbsoluteLocalPath(self):