The code seems pretty clear in its intent, so not sure why the discrepancy. We have two options- Change the documentation or change the code to be in accordance with the documentation, however this may break some extensions.

Seems the documentation is inconsistent. Some places do specify that cancellation is allowed for other stages. Will update the docs.

updating docs to reflect code sgtm.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ba9fcdb8648ddcf577f121ba908df84888bd8056

commit ba9fcdb8648ddcf577f121ba908df84888bd8056
Author: Karan Bhatia <karandeepb@chromium.org>
Date: Tue Aug 29 03:56:49 2017

WebRequest API Docs: Clarify the stages in which request can be cancelled.

The current documentation for the Web Request API is a bit unclear on the
network request stages in which a network request can be cancelled. Update the
docs to reflect that a network request can be cancelled in the onBeforeRequest,
onBeforeSendHeaders, onHeadersReceived and onAuthRequired stages.

BUG= 695303 

Change-Id: I6464a21aa5049d17d70027c7a5c8748e7ba1e81b
Reviewed-on: https://chromium-review.googlesource.com/639234
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498017}
[modify] https://crrev.com/ba9fcdb8648ddcf577f121ba908df84888bd8056/chrome/common/extensions/docs/templates/intros/webRequest.html
[modify] https://crrev.com/ba9fcdb8648ddcf577f121ba908df84888bd8056/extensions/common/api/web_request.json