Add --override-os-version to telemetry for decorators

telemetry/telemetry/testing/run_tests.py

Index: telemetry/telemetry/testing/run_tests.py
diff --git a/telemetry/telemetry/testing/run_tests.py b/telemetry/telemetry/testing/run_tests.py
index fec970adfd11834b3f26e2075671ebcc0e21ee4d..67b16a386c2d34d06dd387221e75f89a5c7bc348 100644
--- a/telemetry/telemetry/testing/run_tests.py
+++ b/telemetry/telemetry/testing/run_tests.py
@@ -69,6 +69,9 @@ class RunTestsCommand(command_line.OptparseCommand):
                       action='append', help=(
                           'Globs of test names to skip (defaults to '
                           '%(default)s).'))
+    parser.add_option('--override-os-version', type=str, default="",
+                      help='Provide a custom OS Version to be used when '
+                           'matching test decorators.')
     typ.ArgumentParser.add_option_group(parser,
                                         "Options for running the tests",
                                         running=True,
@@ -137,6 +140,9 @@ class RunTestsCommand(command_line.OptparseCommand):
       possible_browser = browser_finder.FindBrowser(args)
       platform = possible_browser.platform
 
+    if args.override_os_version:
+      platform._platform_backend.override_os_version = args.override_os_version
+
     fetch_reference_chrome_binary = False
     # Fetch all binaries needed by telemetry before we run the benchmark.
     if possible_browser and possible_browser.browser_type == 'reference':