In update-w3c-test-expectations, stage all new files before committing.

Currently, w3c-test-autoroller is blocked on importing
because new baseline files are not added and committed
before uploading.  Example of a build where this happens:
https://build.chromium.org/p/chromium.infra.cron/builders/w3c-test-autoroller/builds/7732.

Purpose: This CL would unblock this issue on w3c-test-autoroller.

This would be unnecessary (and could be reverted) if either of
http://crrev.com/2397573002 or http://crrev.com/2396433004 is committed.

BUG=621599

Review-Url: https://codereview.chromium.org/2394913002
Cr-Commit-Position: refs/heads/master@{#423728}

third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py

@@ -83,6 +83,9 @@ class RebaselineCL(AbstractParallelRebaselineCommand):
 
         if options.dry_run:
             return
+        # NOTE(qyearsley): If this is changed to stage all new files with git,
+        # e.g. if update_scm is not False, then update_w3c_test_expectations.py
+        # should be changed to not call git add --all.
         self.rebaseline(options, test_prefix_list, update_scm=False)
 
     def _filter_existing(self, test_prefix_list):

third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations.py

@@ -316,6 +316,9 @@ class W3CExpectationsLineAdder(object):
                 '--no-trigger-jobs',
                 '--only-changed-tests',
             ] + tests_to_rebaseline)
+            # NOTE(qyearsley): If rebaseline-cl is changed to stage all new files
+            # with git, then this would be unnecessary and should be removed.
+            self.host.executive.run_command(['git', 'add', '--all'])
         return tests_results
 
     def get_modified_existing_tests(self):