Fixes mac notarization that C79 update broke.

Fixes brave/brave-browser#7191

When creating config use ChromiumCodeSigningConfig instead of the now
bare bones model.CodeSignConfig.

Chromium change:

https://chromium.googlesource.com/chromium/src/+/c04be46a5d8ecbc2cf4d565a453d11212a9fd124

commit c04be46a5d8ecbc2cf4d565a453d11212a9fd124
Author: Robert Sesek <rsesek@chromium.org>
Date:   Wed Sep 18 00:57:47 2019 +0000

    macOS Signing Scripts: Split the config class and expand the hierarchy.

    The base CodeSignConfig is now in a normal Python module and it has
    unimplemented properties. The build-system-injected variables inherit
    from that for the BuildPropsCodeSignConfig. A new
    ChromiumCodeSignConfig class is created to set the signing properties
    for Chromium, to mirror the InternalCodeSignConfig for Google Chrome
    builds. Both ChromiumConfig and InternalCodeSignConfig inherit from
    the BuildPropsCodeSignConfig.

    This makes it easier to use the signing module without needing to
    process a config file.

build/mac/notarize_dmg_pkg.py

@@ -24,7 +24,7 @@ packaging_signing_path = os.path.realpath(os.path.dirname(os.path.realpath(__fil
 sys.path.append(packaging_signing_path)
 
 # Import the entire module to avoid circular dependencies in the functions
-from signing import config, commands, model, notarize, pipeline, signing  # noqa: E402
+from signing import chromium_config, config, commands, model, notarize, pipeline, signing  # noqa: E402
 from signing_helper import GetBraveSigningConfig
 
 def run_command(args, **kwargs):
@@ -47,7 +47,7 @@ def create_config(config_args, development, mac_provisioning_profile):
     Returns:
         An instance of |model.CodeSignConfig|.
     """
-    config_class = config.CodeSignConfig
+    config_class = ChromiumCodeSignConfig
 
     if development: