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 <[email protected]>
Commit-Queue: Julian Pastarmov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#678407}
2 files changed