Change the default --retry-limit for telemetry*unittests.

Previously you had to pass the flag to get failures to be retried;
now we will retry failures by default if you run all of the tests
(i.e., do not specify any tests explicitly on the command line).

You can still pass --retry-limit=0 to turn off the retries and
--retry-limit=3 (or whatever) to force them on.

TBR=tonyg@chromium.org
BUG=402089

Review URL: https://codereview.chromium.org/468363002

Cr-Commit-Position: refs/heads/master@{#289554}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289554 0039d316-1c4b-4281-b951-d872f2087c98

tools/telemetry/telemetry/unittest/run_tests.py

@@ -126,8 +126,8 @@ class RunTestsCommand(command_line.OptparseCommand):
                       dest='run_disabled_tests',
                       action='store_true', default=False,
                       help='Ignore @Disabled and @Enabled restrictions.')
-    parser.add_option('--retry-limit', type='int', default=0,
-                      help='Retry each failure up to N times (default %default)'
+    parser.add_option('--retry-limit', type='int',
+                      help='Retry each failure up to N times'
                            ' to de-flake things.')
     json_results.AddOptions(parser)
 
@@ -136,6 +136,11 @@ class RunTestsCommand(command_line.OptparseCommand):
     if args.verbosity == 0:
       logging.getLogger().setLevel(logging.WARN)
 
+    # We retry failures by default unless we're running a list of tests
+    # explicitly.
+    if args.retry_limit is None and not args.positional_args:
+      args.retry_limit = 3
+
     try:
       possible_browser = browser_finder.FindBrowser(args)
     except browser_finder.BrowserFinderException, ex: