Use = in flags from flags_compatibility.py.

We need this because there is a current bug in Abseil flags.

Bug: chromium:1051927
Change-Id: I0446fadad709abf4aecda1ff5c0d4f7f6218879e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171880
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30910}
diff --git a/tools_webrtc/flags_compatibility.py b/tools_webrtc/flags_compatibility.py
index 8265537..d3570a5 100644
--- a/tools_webrtc/flags_compatibility.py
+++ b/tools_webrtc/flags_compatibility.py
@@ -21,7 +21,7 @@
 
   test_command = _ForcePythonInterpreter(unrecognized_args)
   if args.isolated_script_test_perf_output:
-    test_command += ['--isolated_script_test_perf_output',
+    test_command += ['--isolated_script_test_perf_output=' +
                      args.isolated_script_test_perf_output]
   logging.info('Running %r', test_command)