Ensure manifest exists in Port._wpt_manifest.

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/base.py

 # Copyright (C) 2010 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 731 matching lines @@
                 tests.append(file_path)
                 continue
             urls = self._wpt_manifest().file_path_to_url_paths(match.group(1))
             for url in urls:
                 tests.append(file_path[0:match.start(1)] + url)
         return tests
 
     @memoized
     def _wpt_manifest(self):
         manifest_path = self._filesystem.join(self.layout_tests_dir(), 'external', 'wpt', 'MANIFEST.json')
+        if not self._filesystem.exists(manifest_path):
+            _log.error('Manifest not found at %s. See http://crbug.com/698294', manifest_path)
+            return WPTManifest('{}')
         return WPTManifest(self._filesystem.read_text_file(manifest_path))
 
     def is_slow_wpt_test(self, test_file):
         match = re.match(r'external/wpt/(.*)', test_file)
         if not match:
             return False
         return self._wpt_manifest().is_slow_test(match.group(1))
 
     ALL_TEST_TYPES = ['audio', 'harness', 'pixel', 'ref', 'text', 'unknown']
 
@@ skipping 873 matching lines @@
 
     def __init__(self, base, args, reference_args=None):
         self.name = base
         self.base = base
         self.args = args
         self.reference_args = args if reference_args is None else reference_args
         self.tests = set()
 
     def __repr__(self):
         return "PhysicalTestSuite('%s', '%s', %s, %s)" % (self.name, self.base, self.args, self.reference_args)