The iframe ancestor check was also removed:
https://github.com/whatwg/fullscreen/commit/9592913bbeca3a9d029274f310eb27ad8003622c

This should not block  issue 383813 , because my plan is to ship the unprefixed API with this restriction in place. This is because Edge and Firefox have it, and this will mean that it's less work for them to also ship the unprefixed API. This can be fixed later, or we can change the spec.

Note that for UserActivationV2, two "sibling" subframes can never share a user activation, so allowing both subframes' fullscreen requests is possible only when each subframe has its own activation occurred within few seconds of each other.

Because of #c4, the test fullscreen/api/element-request-fullscreen-two-iframes.html is failing for UserActivationV2.  I will try to fix the test in a trivial click-target fixing.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/69a828594a7b4e83a35eaf0b4be417434880b588

commit 69a828594a7b4e83a35eaf0b4be417434880b588
Author: Mustaq Ahmed <mustaq@google.com>
Date: Mon Jan 15 15:50:10 2018

Fixed the click coordinate in a fullscreen layouttest.

Bug:  784999 ,  627792 
Change-Id: I120d97f3d9b248bfe3cb22f1ff89b43101fc17b4
Reviewed-on: https://chromium-review.googlesource.com/861748
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529288}
[modify] https://crrev.com/69a828594a7b4e83a35eaf0b4be417434880b588/third_party/WebKit/LayoutTests/fullscreen/api/element-request-fullscreen-two-iframes.html

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/79f09add547150c32f408d7b54e96697051a8175

commit 79f09add547150c32f408d7b54e96697051a8175
Author: Dave Tapuska <dtapuska@chromium.org>
Date: Wed Aug 29 16:08:07 2018

Sync fullscreen Element Ready steps with spec.

I discovered the fullscreen element ready steps didn't quite match
the spec. Changes  https://github.com/whatwg/fullscreen/issues/45 and
https://github.com/whatwg/fullscreen/commit/9592913bbeca3a9d029274f310eb27ad8003622c
weren't reflected in our implementation. I've modified the tests to match.

BUG= 627792 

Change-Id: Ia0cd0700ea05248d14225d551f3e5287be2b99ef
Reviewed-on: https://chromium-review.googlesource.com/1191728
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587138}
[modify] https://crrev.com/79f09add547150c32f408d7b54e96697051a8175/third_party/WebKit/LayoutTests/TestExpectations
[modify] https://crrev.com/79f09add547150c32f408d7b54e96697051a8175/third_party/WebKit/LayoutTests/external/wpt/fullscreen/api/element-ready-check-containing-iframe-manual.html
[modify] https://crrev.com/79f09add547150c32f408d7b54e96697051a8175/third_party/WebKit/LayoutTests/external/wpt/fullscreen/api/element-ready-check-fullscreen-element-sibling-manual.html
[delete] https://crrev.com/468ce86a010ce8313e6b92a4ffb8a4a386189342/third_party/WebKit/LayoutTests/external/wpt/fullscreen/api/element-ready-check-fullscreen-iframe-child-manual-expected.txt
[delete] https://crrev.com/468ce86a010ce8313e6b92a4ffb8a4a386189342/third_party/WebKit/LayoutTests/external/wpt/fullscreen/api/element-ready-check-iframe-child-manual-expected.txt
[modify] https://crrev.com/79f09add547150c32f408d7b54e96697051a8175/third_party/WebKit/LayoutTests/external/wpt/fullscreen/api/element-request-fullscreen-non-top-manual.html
[delete] https://crrev.com/468ce86a010ce8313e6b92a4ffb8a4a386189342/third_party/WebKit/LayoutTests/fullscreen/api/element-request-fullscreen-two-elements.html
[delete] https://crrev.com/468ce86a010ce8313e6b92a4ffb8a4a386189342/third_party/WebKit/LayoutTests/fullscreen/api/element-request-fullscreen-two-iframes.html
[modify] https://crrev.com/79f09add547150c32f408d7b54e96697051a8175/third_party/WebKit/LayoutTests/fullscreen/full-screen-twice-expected.txt
[modify] https://crrev.com/79f09add547150c32f408d7b54e96697051a8175/third_party/WebKit/LayoutTests/fullscreen/full-screen-twice.html
[modify] https://crrev.com/79f09add547150c32f408d7b54e96697051a8175/third_party/blink/renderer/core/fullscreen/fullscreen.cc