[infra] Fix v8gen.py

Looks like this was broken by the renaming in
https://chromium-review.googlesource.com/c/v8/v8/+/2627309

Bug: v8:11361
Change-Id: I07c74a847171070a6a9296f3f6e1ef1f45002c66
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2651700Reviewed-by: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72384}

tools/dev/v8gen.py

@@ -146,7 +146,7 @@ class GenerateGnArgs(object):
       self._options.builder = self._options.outdir
 
     # Check for builder/config in mb config.
-    if self._options.builder not in self._mbw.masters[self._options.master]:
+    if self._options.builder not in self._mbw.builder_groups[self._options.master]:
       print('%s does not exist in %s for %s' % (
           self._options.builder, CONFIG, self._options.master))
       return 1
@@ -292,10 +292,10 @@ class GenerateGnArgs(object):
     self._mbw.ParseArgs(['lookup', '-f', CONFIG])
     self._mbw.ReadConfigFile()
 
-    if not self._options.master in self._mbw.masters:
+    if not self._options.master in self._mbw.builder_groups:
       print('%s not found in %s\n' % (self._options.master, CONFIG))
       print('Choose one of:\n%s\n' % (
-          '\n'.join(sorted(self._mbw.masters.keys()))))
+          '\n'.join(sorted(self._mbw.builder_groups.keys()))))
       return 1
 
     return self._options.func()