I2S: https://groups.google.com/a/chromium.org/d/msg/blink-dev/LnqwTCiT9Gs/tO0IBO4PAwAJ

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/66bf5b52c2ad3611ce284c675f2a51181d81c19a

commit 66bf5b52c2ad3611ce284c675f2a51181d81c19a
Author: kdillon <kdillon@chromium.org>
Date: Wed Jan 02 22:08:30 2019

Disallowing sync XHR in pagedimissal.

Disallow sync XHR during page dismissal when the page is being navigated
away or closed by the user. Sync XHR is already on the deprecation path and
its usage hurts end user experience.

Rolling this out behind a base::Feature to help mitigate risk. This feature
is disabled by default.

Bug: 827324
Change-Id: I32cb71c3896281c1311ee32de3e7e434ba80322b
Reviewed-on: https://chromium-review.googlesource.com/c/1336914
Commit-Queue: Katie Dillon <kdillon@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619511}
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/chrome/test/data/prerender/prerender_loader_with_unload.html
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/content/child/runtime_features.cc
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/testing/variations/fieldtrial_testing_config.json
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/third_party/blink/common/features.cc
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/third_party/blink/public/common/features.h
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/third_party/blink/public/platform/web_runtime_features.h
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/third_party/blink/renderer/core/xmlhttprequest/xml_http_request.cc
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/third_party/blink/renderer/platform/exported/web_runtime_features.cc
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/third_party/blink/renderer/platform/runtime_enabled_features.json5
[modify] https://crrev.com/66bf5b52c2ad3611ce284c675f2a51181d81c19a/tools/metrics/histograms/enums.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/049fce1f09cb3f0e73a5bac56619a0b60dd1bfe2

commit 049fce1f09cb3f0e73a5bac56619a0b60dd1bfe2
Author: kdillon <kdillon@chromium.org>
Date: Fri Jan 18 03:09:43 2019

Adding use counter for forbidden synchronous XHR in page dismissal.

Adding a new use counter and histogram for tracking when sync xhr is
disallowed in page dismissal.

Bug: 827324
Change-Id: I25afaad4b87d06cc0b46b9fb6748f8505d3951bf
Reviewed-on: https://chromium-review.googlesource.com/c/1412015
Commit-Queue: Katie Dillon <kdillon@chromium.org>
Reviewed-by: Shubhie Panicker <panicker@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623981}
[modify] https://crrev.com/049fce1f09cb3f0e73a5bac56619a0b60dd1bfe2/third_party/blink/public/platform/web_feature.mojom
[modify] https://crrev.com/049fce1f09cb3f0e73a5bac56619a0b60dd1bfe2/third_party/blink/renderer/core/xmlhttprequest/xml_http_request.cc
[modify] https://crrev.com/049fce1f09cb3f0e73a5bac56619a0b60dd1bfe2/tools/metrics/histograms/enums.xml