Reland "Add wrapper to normalize flags."

This is a reland of 642a49d1eb20b8c5744e745de79ddb585e0f7472

The change has the same effect but is now implemented through mb, rather than specifying a 'script', so that Android's special handling is not skipped.

Original change's description:
> Add wrapper to normalize flags.
>
> Bug: None
> Change-Id: I9d43602cc66198a29dbc0e7586d948ee76c5ec84
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145204
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#28532}

Bug: webrtc:10616
Change-Id: I60ebd4891dbe8de18c653f8af88181ea966307de
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145409
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28572}

tools_webrtc/mb/mb_unittest.py

@@ -453,9 +453,11 @@ class UnitTest(unittest.TestCase):
     self.assertEqual(files, [
         '../../.vpython',
         '../../testing/test_env.py',
+        '../../tools_webrtc/flags_compatibility.py',
         'base_unittests',
     ])
     self.assertEqual(command, [
+        '../../tools_webrtc/flags_compatibility.py',
         '../../testing/test_env.py',
         './base_unittests',
         '--asan=0',