Add ability to find unused flag with list_flags.py.

Add a new "-u" switch to list_flags.py to search for unused flags. It
checks the obvious files that should reference flags.

Change-Id: I0f9b050b2adcafd0a612bae93f2e4ef08cdf0e7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416851
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808477}

tools/flags/list_flags.py

@@ -84,6 +84,21 @@ def resolve_owners(flags):
   return new_flags
 
 
+def find_unused(flags):
+  FLAG_FILES = [
+      'chrome/browser/about_flags.cc',
+      'ios/chrome/browser/flags/about_flags.mm',
+  ]
+  flag_files_data = [open(f, 'rb').read().decode('utf-8') for f in FLAG_FILES]
+  unused_flags = []
+  for flag in flags:
+    # Search for the name in quotes.
+    needle = '"%s"' % flag['name']
+    if not any([needle in data for data in flag_files_data]):
+      unused_flags.append(flag)
+  return unused_flags
+
+
 def print_flags(flags, verbose):
   """Prints the supplied list of flags.
 
@@ -119,6 +134,7 @@ def main():
   group = parser.add_mutually_exclusive_group()
   group.add_argument('-n', '--never-expires', action='store_true')
   group.add_argument('-e', '--expired-by', type=int)
+  group.add_argument('-u', '--find-unused', action='store_true')
   parser.add_argument('-v', '--verbose', action='store_true')
   parser.add_argument('--testonly', action='store_true')
   args = parser.parse_args()
@@ -131,6 +147,8 @@ def main():
     flags = keep_expired_by(flags, args.expired_by)
   if args.never_expires:
     flags = keep_never_expires(flags)
+  if args.find_unused:
+    flags = find_unused(flags)
   flags = resolve_owners(flags)
   print_flags(flags, args.verbose)