Move temporary option for SameSite into Driver.cmd_line()

The previous method of temporary option for SameSite affected
flag-specific expectatations because the temporary option was treated
as the flag-specific name. This broke local rebaselines of tests.

Move the temporary option into Driver.cmd_line() to prevent it from
affecting flag-specific feature.

Bug: 961439
Change-Id: I1b2c30feebb42a0a701b2085b63b0fde4ab2ea93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914196Reviewed-by: Lily Chen <chlily@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714996}

third_party/blink/tools/blinkpy/web_tests/port/driver.py

@@ -464,6 +464,13 @@ class Driver(object):
         cmd.extend(self._port.additional_driver_flags())
         if self._port.get_option('enable_leak_detection'):
             cmd.append('--enable-leak-detection')
+
+        # Run tests with the new SameSite cookie behavior by default.
+        # By appending the features to --enable-features, they will be enabled if
+        # they are not also explicitly disabled (as base::FeatureList disables a
+        # feature that appears in both --disable-features and --enable-features).
+        cmd.append('--enable-features=SameSiteByDefaultCookies,CookiesWithoutSameSiteMustBeSecure')
+
         cmd.extend(per_test_args)
         cmd = coalesce_repeated_switches(cmd)
         cmd.append('-')

third_party/blink/tools/blinkpy/web_tests/run_web_tests.py

@@ -46,13 +46,6 @@ _log = logging.getLogger(__name__)
 def main(argv, stderr):
     options, args = parse_args(argv)
 
-    # Run tests with the new SameSite cookie behavior by default.
-    # By appending the features to --enable-features, they will be enabled if
-    # they are not also explicitly disabled (as base::FeatureList disables a
-    # feature that appears in both --disable-features and --enable-features).
-    samesite_feature_names = ['SameSiteByDefaultCookies', 'CookiesWithoutSameSiteMustBeSecure']
-    options.additional_driver_flag.append('--enable-features=' + ','.join(samesite_feature_names))
-
     if options.platform and 'test' in options.platform and not 'browser_test' in options.platform:
         # It's a bit lame to import mocks into real code, but this allows the user
         # to run tests against the test platform interactively, which is useful for