Add tree status check to PDFium CQ.

This CL adds in the tree status check so the CQ will not commit if the waterfall
closes the PDFium tree.

BUG=chromium:607625

Review-Url: https://codereview.chromium.org/1937943002
diff --git a/infra/config/cq.cfg b/infra/config/cq.cfg
index 0a494e8..90dbfc8 100644
--- a/infra/config/cq.cfg
+++ b/infra/config/cq.cfg
@@ -15,6 +15,10 @@
      committer_list: "project-pdfium-committers"
   }
 
+  tree_status {
+    tree_status_url: "https://pdfium-status.appspot.com"
+  }
+
   try_job {
     buckets {
       name: "master.tryserver.client.pdfium"