[WPT export] Only search the master branch on Gerrit

Bug: 935208
Change-Id: Ie5e73c7fd01c46d7f033c9f2ed5522c35926bd3f
Reviewed-on: https://chromium-review.googlesource.com/c/1485492
Auto-Submit: Robert Ma <robertma@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635165}

third_party/blink/tools/blinkpy/w3c/gerrit.py

@@ -58,7 +58,7 @@ class GerritAPI(object):
         return cl
 
     def query_exportable_open_cls(self, limit=200):
-        path = ('/changes/?q=project:\"chromium/src\"+status:open'
+        path = ('/changes/?q=project:\"chromium/src\"+branch:master+status:open'
                 '&o=CURRENT_FILES&o=CURRENT_REVISION&o=COMMIT_FOOTERS'
                 '&o=DETAILED_ACCOUNTS&o=DETAILED_LABELS&n={}').format(limit)
         # The underlying host.web.get_binary() automatically retries until it