diff --git a/chrome/BUILD.gn b/chrome/BUILD.gn
index 1db8721f0d9a00..73f827c8c0e054 100644
--- a/chrome/BUILD.gn
+++ b/chrome/BUILD.gn
@@ -1702,6 +1702,7 @@ if (enable_resource_whitelist_generation) {
       "-o",
       rebase_path(_outfile, root_build_dir),
       "--out-dir=.",
+      "--use-existing-resource-ids",
     ]
   }
 }
diff --git a/tools/resources/generate_resource_whitelist.py b/tools/resources/generate_resource_whitelist.py
index 527ceb484158c9..c4431d687aed95 100755
--- a/tools/resources/generate_resource_whitelist.py
+++ b/tools/resources/generate_resource_whitelist.py
@@ -55,6 +55,22 @@ def _FindResourceIds(header, resource_names):
   return set(res_ids)
 
 
+# TODO(estevenson): Remove this after updating official build scripts.
+def _GetResourceIdsInPragmaWarnings(input):
+   """Returns set of resource ids that are inside unknown pragma warnings
+      for the given input.
+   """
+   used_resources = set()
+   unknown_pragma_warning_pattern = re.compile(
+       'whitelisted_resource_(?P<resource_id>[0-9]+)')
+   for ln in input:
+     match = unknown_pragma_warning_pattern.search(ln)
+     if match:
+       resource_id = int(match.group('resource_id'))
+       used_resources.add(resource_id)
+   return used_resources
+
+
 def main():
   parser = argparse.ArgumentParser(usage=USAGE)
   parser.add_argument(
@@ -66,11 +82,18 @@ def main():
   parser.add_argument(
       '--out-dir', required=True,
       help='The out target directory, for example out/Release')
+  parser.add_argument(
+      '--use-existing-resource-ids', action='store_true', default=False,
+      help='Specifies that the input file already contains resource ids')
 
   args = parser.parse_args()
 
   used_resources = set()
-  used_resources.update([int(resource_id) for resource_id in args.input])
+  if args.use_existing_resource_ids:
+    used_resources.update([int(resource_id) for resource_id in args.input])
+  else:
+    used_resources.update(_GetResourceIdsInPragmaWarnings(args.input))
+
   used_resources |= _FindResourceIds(
       os.path.join(args.out_dir, COMPONENTS_STRINGS_HEADER),
       ARCH_SPECIFIC_RESOURCES)