Add mr_extension as an exception for generated directories.

Bug: 997947
Change-Id: I26f2be79db4137686a37ec4968269c6ec4b72a18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1769849Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Clifford Cheng <cliffordcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690777}

tools/mb/mb.py

@@ -1100,6 +1100,7 @@ class MetaBuildWrapper(object):
       # TODO(https://crbug.com/912946): Remove this if statement.
       if (f == 'angledata/gl_cts/' or  # http://anglebug.com/3827
           (is_msan and f == 'instrumented_libraries_prebuilt/') or
+          f == 'mr_extension/' or # https://crbug.com/997947
           f == 'locales/' or
           f.startswith('nacl_test_data/') or
           f.startswith('ppapi_nacl_tests_libs/') or