The implementation of the flags was landed in r522981. Requesting merge into M64.

Verified in 65.0.3290.0 as follows:

Try to see if document blocking is enabled in the configurations below, using the following steps:
1. Visit http://csreis.github.io/tests/
2. In DevTools Console:
var x = new XMLHttpRequest(); x.onload = function() { console.log("Response: " + x.responseText)}; x.open("GET", "http://www.example.com"); x.send();
3. In about:histograms, check if SiteIsolation.XSD.Browser.Blocked was incremented.

Configurations:
1) No OOPIFs, no blocking (--disable-features=IsolateOrigins,site-per-process).  Should not see any blocking.
2) No OOPIFs, enable blocking (--disable-features=IsolateOrigins,site-per-process --enable-features=CrossSiteDocumentBlockingAlways).  Should see a blocked response.
3) OOPIFs enabled (--site-per-process).  Should see a blocked response.
4) OOPIFs enabled, blocking disabled (--site-per-process --disable-features=CrossSiteDocumentBlockingIfIsolating)

We'll need to merge this to both M64 and the M63 minibranch (3239_84).

Approving merge to  M63 minibranch (3239_84) based on comment #3 and per internal group chat.

Approving for M64. Branch:3282

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c9fcf0daa075daa038f8fea18947e7b0c47e4673

commit c9fcf0daa075daa038f8fea18947e7b0c47e4673
Author: Nasko Oskov <nasko@chromium.org>
Date: Mon Dec 11 21:16:05 2017

Kill switch for cross-site document blocking.

Integrating cross-site document blocking with 2 new content_features.h:

- kCrossSiteDocumentBlockingIfIsolating - kill switch for cross-site
  document blocking.  This feature is enabled by default.

- kCrossSiteDocumentBlockingAlways - a way to force cross-site
  document blocking even if no isolation mode is turned on
  (e.g. even if there is no site-per-process or isolate-origins).
  This feature is disabled by default.

Bug:  786505 ,  793881 
Change-Id: Id0c9a69025fc20f7659b97a96fe70402cc933113
Reviewed-on: https://chromium-review.googlesource.com/818388
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#522981}
Reviewed-on: https://chromium-review.googlesource.com/820452
Cr-Commit-Position: refs/branch-heads/3282@{#148}
Cr-Branched-From: 5fdc0fab22ce7efd32532ee989b223fa12f8171e-refs/heads/master@{#520840}
[modify] https://crrev.com/c9fcf0daa075daa038f8fea18947e7b0c47e4673/content/browser/loader/cross_site_document_blocking_browsertest.cc
[modify] https://crrev.com/c9fcf0daa075daa038f8fea18947e7b0c47e4673/content/browser/loader/cross_site_document_resource_handler.cc
[modify] https://crrev.com/c9fcf0daa075daa038f8fea18947e7b0c47e4673/content/common/site_isolation_policy.cc
[modify] https://crrev.com/c9fcf0daa075daa038f8fea18947e7b0c47e4673/content/common/site_isolation_policy.h
[modify] https://crrev.com/c9fcf0daa075daa038f8fea18947e7b0c47e4673/content/public/common/content_features.cc
[modify] https://crrev.com/c9fcf0daa075daa038f8fea18947e7b0c47e4673/content/public/common/content_features.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d1bd9af24648379c22b20f061a3bf06bb6dc329

commit 3d1bd9af24648379c22b20f061a3bf06bb6dc329
Author: Nasko Oskov <nasko@chromium.org>
Date: Mon Dec 11 21:22:32 2017

Kill switch for cross-site document blocking.

Integrating cross-site document blocking with 2 new content_features.h:

- kCrossSiteDocumentBlockingIfIsolating - kill switch for cross-site
  document blocking.  This feature is enabled by default.

- kCrossSiteDocumentBlockingAlways - a way to force cross-site
  document blocking even if no isolation mode is turned on
  (e.g. even if there is no site-per-process or isolate-origins).
  This feature is disabled by default.

Bug:  786505 ,  793881 
Change-Id: Id0c9a69025fc20f7659b97a96fe70402cc933113
Reviewed-on: https://chromium-review.googlesource.com/818388
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#522981}
Reviewed-on: https://chromium-review.googlesource.com/820673
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/branch-heads/3239_84@{#4}
Cr-Branched-From: 8f51ed0e633e109109762a3deb18a50e8c138819-refs/branch-heads/3239@{#643}
Cr-Branched-From: adb61db19020ed8ecee5e91b1a0ea4c924ae2988-refs/heads/master@{#508578}
[modify] https://crrev.com/3d1bd9af24648379c22b20f061a3bf06bb6dc329/content/browser/loader/cross_site_document_blocking_browsertest.cc
[modify] https://crrev.com/3d1bd9af24648379c22b20f061a3bf06bb6dc329/content/browser/loader/cross_site_document_resource_handler.cc
[modify] https://crrev.com/3d1bd9af24648379c22b20f061a3bf06bb6dc329/content/common/site_isolation_policy.cc
[modify] https://crrev.com/3d1bd9af24648379c22b20f061a3bf06bb6dc329/content/common/site_isolation_policy.h
[modify] https://crrev.com/3d1bd9af24648379c22b20f061a3bf06bb6dc329/content/public/common/content_features.cc
[modify] https://crrev.com/3d1bd9af24648379c22b20f061a3bf06bb6dc329/content/public/common/content_features.h

Approving merge to M63 branch 3239. Please merge ASAP. Thank you.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2b6ff5ec5d927c2d41a50a63d7cb65b566dcefa4

commit 2b6ff5ec5d927c2d41a50a63d7cb65b566dcefa4
Author: Nasko Oskov <nasko@chromium.org>
Date: Wed Dec 13 00:50:29 2017

Kill switch for cross-site document blocking.

Integrating cross-site document blocking with 2 new content_features.h:

- kCrossSiteDocumentBlockingIfIsolating - kill switch for cross-site
  document blocking.  This feature is enabled by default.

- kCrossSiteDocumentBlockingAlways - a way to force cross-site
  document blocking even if no isolation mode is turned on
  (e.g. even if there is no site-per-process or isolate-origins).
  This feature is disabled by default.

Bug:  786505 ,  793881 
TBR=nasko@chromium.org

Change-Id: Id0c9a69025fc20f7659b97a96fe70402cc933113
Reviewed-on: https://chromium-review.googlesource.com/818388
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Original-Original-Commit-Position: refs/heads/master@{#522981}
Reviewed-on: https://chromium-review.googlesource.com/820673
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Original-Commit-Position: refs/branch-heads/3239_84@{#4}
Cr-Original-Branched-From: 8f51ed0e633e109109762a3deb18a50e8c138819-refs/branch-heads/3239@{#643}
Cr-Original-Branched-From: adb61db19020ed8ecee5e91b1a0ea4c924ae2988-refs/heads/master@{#508578}
Reviewed-on: https://chromium-review.googlesource.com/823564
Cr-Commit-Position: refs/branch-heads/3239@{#669}
Cr-Branched-From: adb61db19020ed8ecee5e91b1a0ea4c924ae2988-refs/heads/master@{#508578}
[modify] https://crrev.com/2b6ff5ec5d927c2d41a50a63d7cb65b566dcefa4/content/browser/loader/cross_site_document_blocking_browsertest.cc
[modify] https://crrev.com/2b6ff5ec5d927c2d41a50a63d7cb65b566dcefa4/content/browser/loader/cross_site_document_resource_handler.cc
[modify] https://crrev.com/2b6ff5ec5d927c2d41a50a63d7cb65b566dcefa4/content/common/site_isolation_policy.cc
[modify] https://crrev.com/2b6ff5ec5d927c2d41a50a63d7cb65b566dcefa4/content/common/site_isolation_policy.h
[modify] https://crrev.com/2b6ff5ec5d927c2d41a50a63d7cb65b566dcefa4/content/public/common/content_features.cc
[modify] https://crrev.com/2b6ff5ec5d927c2d41a50a63d7cb65b566dcefa4/content/public/common/content_features.h

Fixed by r522981 and merged to both M64 and M63.

All merges are already done - removing one of the labels to stop nag emails.