To quote myself from an email:

I've wanted this feature myself in the past and would love to see it in action. However, would it be possible to make the transition as nice as in native apps by requesting fullscreen on orientation change? I suspect that this is too late, and that no matter how hard we try there will always be a few wrong frames. The fullscreen transition is currently terrible, but it's because of implementation issues and not because of the model.

How does rotate-to-fullscreen work on Android? It could be that a declarative feature is required to get it right. Ideally, there should be one frame before the rotation that's not fullscreen, one after that is in fullscreen, and then some rotating animation which isn't observable to web contents at all.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/740ed214f4b05ea93bde8a38d889fdc1e5699946

commit 740ed214f4b05ea93bde8a38d889fdc1e5699946
Author: mlamouri <mlamouri@chromium.org>
Date: Mon Jul 25 18:50:00 2016

Allow going fullscreen during a screen orientation change handler.

Currently, fullscreen request is only permitted when it is triggered by
a user gesture. This is allowing allowing a screen orientation change to
request fullscreen.

This CL is taking over https://crrev.com/1818493002 from zqzhang@chromium.org

BUG= 594253 

Review-Url: https://codereview.chromium.org/2175863002
Cr-Commit-Position: refs/heads/master@{#407531}

[add] https://crrev.com/740ed214f4b05ea93bde8a38d889fdc1e5699946/third_party/WebKit/LayoutTests/fullscreen/full-screen-orientation-change.html
[modify] https://crrev.com/740ed214f4b05ea93bde8a38d889fdc1e5699946/third_party/WebKit/Source/core/dom/Fullscreen.cpp
[modify] https://crrev.com/740ed214f4b05ea93bde8a38d889fdc1e5699946/third_party/WebKit/Source/modules/screen_orientation/ScreenOrientationController.cpp
[add] https://crrev.com/740ed214f4b05ea93bde8a38d889fdc1e5699946/third_party/WebKit/Source/platform/ScopedOrientationChangeIndicator.cpp
[add] https://crrev.com/740ed214f4b05ea93bde8a38d889fdc1e5699946/third_party/WebKit/Source/platform/ScopedOrientationChangeIndicator.h
[add] https://crrev.com/740ed214f4b05ea93bde8a38d889fdc1e5699946/third_party/WebKit/Source/platform/ScopedOrientationChangeIndicatorTest.cpp
[modify] https://crrev.com/740ed214f4b05ea93bde8a38d889fdc1e5699946/third_party/WebKit/Source/platform/blink_platform.gypi

\o/ Thanks, Mounir!