Add helper script to output count of filtered tests (for PlzNav bot)

After (an embarassing amount of time spent doing) trial-and-error, it
doesn't seem possible for this type of script to boost the output to
STEP_TEXT so that it shows up on the top-level bot run page
unfortunately. But this at least makes the data available with a click
through to see progress.

Example output here: https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_browser_side_navigation_rel/builds/146/steps/count_filtered_tests_browser_tests%20%28with%20patch%29/logs/json.output

BUG=504374

Review-Url: https://codereview.chromium.org/2353583003
Cr-Commit-Position: refs/heads/master@{#420795}

testing/buildbot/chromium.fyi.json

@@ -1035,6 +1035,24 @@
         },
         "test": "unit_tests"
       }
+    ],
+    "scripts": [
+      {
+        "args": [
+          "browser_tests",
+          "../../testing/buildbot/filters/browser-side-navigation.linux.browser_tests.filter"
+        ],
+        "name": "count_filtered_tests_browser_tests",
+        "script": "count_filtered_tests.py"
+      },
+      {
+        "args": [
+          "content_browsertests",
+          "../../testing/buildbot/filters/browser-side-navigation.linux.content_browsertests.filter"
+        ],
+        "name": "count_filtered_tests_content_browsertests",
+        "script": "count_filtered_tests.py"
+      }
     ]
   },
   "CFI Linux": {

testing/scripts/count_filtered_tests.py

@@ -0,0 +1,89 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import fnmatch
+import json
+import os
+import subprocess
+import sys
+
+
+import common
+
+
+def ParseTestList(test_list_contents):
+  lines = test_list_contents.splitlines()
+
+  all_tests = []
+  fixture = None
+  for line in lines:
+    if '#' in line:
+      line = line[:line.index('#')]
+    line = line.rstrip()
+    if line[0] == ' ':
+      assert fixture
+      all_tests.append(fixture + line.lstrip())
+    else:
+      fixture = line
+
+  return all_tests
+
+
+def LoadFilterList(filter_file):
+  with open(filter_file, 'r') as f:
+    lines = f.readlines()
+
+  all_filters = []
+  for line in lines:
+    if '#' in line:
+      line = line[:line.index('#')]
+    line = line.strip()
+    if not line:
+      continue
+    assert line.startswith('-')  # TODO(scottmg): Handle +s.
+    all_filters.append(line[1:])
+
+  return all_filters
+
+
+def FilterMatchesTest(filter_string, test_string):
+  """Does something close enough to base/strings/pattern.h's MatchPattern() for
+  our purposes here."""
+  return fnmatch.fnmatch(test_string, filter_string)
+
+
+def main_run(args):
+  binary_name = args.args[0]
+  test_filter_file = args.args[1]
+  base_path = os.path.join(args.paths['checkout'], 'out', args.build_config_fs)
+  list_tests_output = subprocess.check_output(
+      [os.path.join(base_path, binary_name), '--gtest_list_tests'])
+  tests = ParseTestList(list_tests_output)
+  negative_filter_list = LoadFilterList(
+      os.path.join(base_path, test_filter_file))
+
+  result = {'valid': True}
+  result['total_tests'] = len(tests)
+  result['disabled_tests'] = len([t for t in tests if '.DISABLED_' in t])
+
+  # Filter out all the tests that aren't being run.
+  for f in negative_filter_list:
+    tests = [t for t in tests if not FilterMatchesTest(f, t)]
+  result['filtered_tests'] = result['total_tests'] - len(tests)
+
+  json.dump(result, args.output)
+  return 0
+
+
+def main_compile_targets(args):
+  # TODO(scottmg): Get the binary name passed here instead of hardcoding.
+  json.dump(['browser_tests', 'content_browsertests'], args.output)
+
+
+if __name__ == '__main__':
+  funcs = {
+    'run': main_run,
+    'compile_targets': main_compile_targets,
+  }
+  sys.exit(common.run_script(sys.argv[1:], funcs))