[WPT Export] Add merge_method: rebase to WPTGitHub.merge_pull_request

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

 # 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 base64
 import json
 import logging
 import os
 import sys
 import urllib2
@@ skipping 69 matching lines @@
     def in_flight_pull_requests(self):
         path = '/search/issues?q=repo:w3c/web-platform-tests%20is:open%20type:pr%20label:{}'.format(EXPORT_LABEL)
         data, status_code = self.request(path, method='GET')
         if status_code == 200:
             return data['items']
         else:
             raise Exception('Non-200 status code (%s): %s' % (status_code, data))
 
     def merge_pull_request(self, pull_request_number):
         path = '/repos/w3c/web-platform-tests/pulls/%d/merge' % pull_request_number
-        body = {}
+        body = {
+            'merge_method': 'rebase',
+        }
         data, status_code = self.request(path, method='PUT', body=body)
 
         if status_code == 200:
             return data
         else:
             raise Exception('PR could not be merged: %d' % pull_request_number)
 
     def delete_remote_branch(self, remote_branch_name):
         # TODO(jeffcarp): Unit test this method
         path = '/repos/w3c/web-platform-tests/git/refs/heads/%s' % remote_branch_name
         data, status_code = self.request(path, method='DELETE')
 
         if status_code != 204:
             # TODO(jeffcarp): Raise more specific exception (create MergeError class?)
             raise Exception('Received non-204 status code attempting to delete remote branch: {}'.format(status_code))
 
         return data