My version of this is starting to come together here: https://codereview.chromium.org/1797363002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f

commit d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f
Author: nick <nick@chromium.org>
Date: Thu Mar 31 19:52:44 2016

Introduce --top-document-isolation mode, a new process model wherein the top
document is process separated from its subframes when possible, but the subframes
are all housed in a single renderer process.

Currently this mode only shares the subframe process within a browsing instance:
usually, that will mean sharing the subframe process within a tab. In follow up patches
we can extend the sharing so that we might share subframe RenderProcessHosts
among unrelated BrowsingInstances, when legal.

BUG=594217
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/1797363002

Cr-Commit-Position: refs/heads/master@{#384364}

[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/chrome/app/generated_resources.grd
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/chrome/browser/about_flags.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/browsing_instance.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/browsing_instance.h
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/frame_host/navigator_impl_unittest.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/frame_host/render_frame_host_impl.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/frame_host/render_frame_host_impl.h
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/frame_host/render_frame_host_manager.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/frame_host/render_frame_host_manager.h
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/renderer_host/render_process_host_impl.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/site_instance_impl.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/site_instance_impl.h
[add] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/browser/top_document_isolation_browsertest.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/common/site_isolation_policy.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/common/site_isolation_policy.h
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/content_tests.gypi
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/public/common/content_switches.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/public/common/content_switches.h
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/content/test/content_browser_test_utils_internal.cc
[modify] https://crrev.com/d5bbd0b067cf0c1c8e40c7b5c2d1a1a39c0db10f/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a9f3ad7a01940804a4741f02721b459add84a9e8

commit a9f3ad7a01940804a4741f02721b459add84a9e8
Author: nick <nick@chromium.org>
Date: Thu Apr 07 03:07:49 2016

SiteInstance unittest improvements
 - Add a new unittest for GetDefaultSubframeSiteInstance.
 - Rewrite tests to not depend on inheritance; BrowsingInstance calls
   the ctor of SiteInstanceImpl directly, and this made it hard to
   do deletion counters for the defaul subframe siteinstance using
   the inheritance tricks.
 - Rework delete counters, using existing mechanisms in the API.
 - mark SiteInstanceImpl/BrowsingInstance final, since we can.

BUG=594217

Review URL: https://codereview.chromium.org/1845233003

Cr-Commit-Position: refs/heads/master@{#385639}

[modify] https://crrev.com/a9f3ad7a01940804a4741f02721b459add84a9e8/content/browser/browsing_instance.h
[modify] https://crrev.com/a9f3ad7a01940804a4741f02721b459add84a9e8/content/browser/site_instance_impl.h
[modify] https://crrev.com/a9f3ad7a01940804a4741f02721b459add84a9e8/content/browser/site_instance_impl_unittest.cc