Refactor WPT Export to ensure only one PR in flight at a time

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

+class MockWPTGitHub(object):
+
+    def __init__(self, pull_requests, unsuccessful_merge=False):
+        self.pull_requests = pull_requests
+        self.unsuccessful_merge = unsuccessful_merge
+        self.calls = []
+        self.pull_requests_created = []
+
+    def in_flight_pull_requests(self):
+        self.calls.append('in_flight_pull_requests')
+        return self.pull_requests
+
+    def merge_pull_request(self, number):
+        self.calls.append('merge_pull_request')
+        if self.unsuccessful_merge:
+            raise Exception('PR could not be merged: %d' % number)
+
+    def create_pr(self, local_branch_name, desc_title, body):
+        self.calls.append('create_pr')
+
+        assert local_branch_name
+        assert desc_title
+        assert body
+
+        self.pull_requests_created.append((local_branch_name, desc_title, body))
+
+        return {}