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/+/1705799
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678407}
diff --git a/PRESUBMIT_test_mocks.py b/PRESUBMIT_test_mocks.py
index ed6a750..f641009 100644
--- a/PRESUBMIT_test_mocks.py
+++ b/PRESUBMIT_test_mocks.py
@@ -220,6 +220,10 @@
     """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):