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}
diff --git a/tools_webrtc/mb/mb.py b/tools_webrtc/mb/mb.py
index 43b9cb4..9628a67 100755
--- a/tools_webrtc/mb/mb.py
+++ b/tools_webrtc/mb/mb.py
@@ -822,15 +822,19 @@
       '../../testing/test_env.py',
     ]
 
+    if test_type == 'raw':
+      cmdline.append('../../tools_webrtc/flags_compatibility.py')
+      extra_files.append('../../tools_webrtc/flags_compatibility.py')
+
     must_retry = False
     if test_type == 'script':
-      cmdline = ['../../' + self.ToSrcRelPath(isolate_map[target]['script'])]
+      cmdline += ['../../' + self.ToSrcRelPath(isolate_map[target]['script'])]
     elif is_android:
-      cmdline = ['../../build/android/test_wrapper/logdog_wrapper.py',
-                 '--target', target,
-                 '--logdog-bin-cmd', '../../bin/logdog_butler',
-                 '--logcat-output-file', '${ISOLATED_OUTDIR}/logcats',
-                 '--store-tombstones']
+      cmdline += ['../../build/android/test_wrapper/logdog_wrapper.py',
+                  '--target', target,
+                  '--logdog-bin-cmd', '../../bin/logdog_butler',
+                  '--logcat-output-file', '${ISOLATED_OUTDIR}/logcats',
+                  '--store-tombstones']
     else:
       if isolate_map[target].get('use_webcam', False):
         cmdline.append('../../tools_webrtc/ensure_webcam_is_running.py')