You need to configure cq.cfg (https://code.google.com/p/chromium/codesearch#chromium/src/third_party/pdfium/infra/config/cq.cfg&q=cq.cfg%20pdfium&sq=package:chromium&l=5) to look for tree status, see e.g. https://code.google.com/p/chromium/codesearch#chromium/src/infra/config/cq.cfg&q=cq.cfg&sq=package:chromium&type=cs&l=1 :

  tree_status {
    tree_status_url: "https://chromium-status.appspot.com"
  }

(of course use pdfium tree status URL; you may need to set up one)

The following revision refers to this bug:
  https://pdfium.googlesource.com/pdfium.git/+/a86d113be692153d7707da377d11d4f09cb12c9b

commit a86d113be692153d7707da377d11d4f09cb12c9b
Author: dsinclair <dsinclair@chromium.org>
Date: Mon May 02 14:42:03 2016

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

[modify] https://crrev.com/a86d113be692153d7707da377d11d4f09cb12c9b/infra/config/cq.cfg

Does the CQ need to be restarted to pickup this change or should it now be live?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a7b49810b22ebaed37a9290809dd6f21ae6bf453

commit a7b49810b22ebaed37a9290809dd6f21ae6bf453
Author: ochang <ochang@chromium.org>
Date: Mon May 02 20:06:38 2016

Roll PDFium 76c5379..a86d113

https://pdfium.googlesource.com/pdfium.git/+log/76c5379..a86d113

BUG=62400,589955, 607625 , 596947 , 602505 , 607739 

TBR=dsinclair@chromium.org

Review-Url: https://codereview.chromium.org/1938013002
Cr-Commit-Position: refs/heads/master@{#391037}

[modify] https://crrev.com/a7b49810b22ebaed37a9290809dd6f21ae6bf453/DEPS

See question in #4.

The change should be live.