I see in the console:

Uncaught SecurityError: Failed to execute 'sendBeacon' on 'Navigator': sendBeacon() with a Blob whose type is not any of the CORS-safelisted values for the Content-Type request header is disabled temporarily. See  http://crbug.com/490015  for details.

Looks like this is a dup of issue 724929.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8c84af9b87477316ffe0d5d09b26d37e700854f4

commit 8c84af9b87477316ffe0d5d09b26d37e700854f4
Author: Yutaka Hirano <yhirano@chromium.org>
Date: Tue Oct 02 10:12:11 2018

Update beacon WPTs

 - Fix beacon-common.sub.js and so that it can handle exceptions thrown
   from sendBeacon.
 - Simplify beacon-common.sub.js.
 - Remove navigate.iFrame.sub.html. Replace it with scripts in the
   main page.

Bug: 876670, 626703, 848275, 876669
Change-Id: I847dec3a71b27c76df156326637be243876daaab
Reviewed-on: https://chromium-review.googlesource.com/1237796
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595780}
[modify] https://crrev.com/8c84af9b87477316ffe0d5d09b26d37e700854f4/third_party/WebKit/LayoutTests/TestExpectations
[modify] https://crrev.com/8c84af9b87477316ffe0d5d09b26d37e700854f4/third_party/WebKit/LayoutTests/external/wpt/beacon/beacon-common.sub.js
[add] https://crrev.com/8c84af9b87477316ffe0d5d09b26d37e700854f4/third_party/WebKit/LayoutTests/external/wpt/beacon/beacon-cors.sub.window-expected.txt
[modify] https://crrev.com/8c84af9b87477316ffe0d5d09b26d37e700854f4/third_party/WebKit/LayoutTests/external/wpt/beacon/beacon-cors.sub.window.js
[add] https://crrev.com/8c84af9b87477316ffe0d5d09b26d37e700854f4/third_party/WebKit/LayoutTests/external/wpt/beacon/beacon-navigate-expected.txt
[add] https://crrev.com/8c84af9b87477316ffe0d5d09b26d37e700854f4/third_party/WebKit/LayoutTests/external/wpt/beacon/beacon-redirect.window-expected.txt
[modify] https://crrev.com/8c84af9b87477316ffe0d5d09b26d37e700854f4/third_party/WebKit/LayoutTests/external/wpt/beacon/beacon-redirect.window.js
[delete] https://crrev.com/c692675544359106e9a935a68e65ddaee4a6d6d8/third_party/WebKit/LayoutTests/external/wpt/beacon/navigate.iFrame.sub.html