While viewing WebVR content in presentation mode, put device into a Daydream headset, and see the same site.

For clarification, this is about being able to navigate back from WebVR presentation to a 2D web page.  This should be solved by having navigation/menu in Chrome VR; there may be additional work to make this pretty, but that doesn't need to be high priority.

I want to exit a WebVR experience (presentation mode) and view the 2D parent site, while viewing WebVR content in Chrome VR.

(updated bug title to better capture scope)

TODO: make child bugs for desired UX:

* If user manually triggered WebVR presentation mode, back button should exit presentation mode.

* If page automatically triggered WebVR presentation mode on load, back button should do history back.

This technical inconsistency will hopefully map more accurately to user perception / expectations.

Do you consider presentation on headset insertion to be manual or automatic? Should we navigate back, or stop presenting?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087

commit 5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087
Author: tiborg <tiborg@chromium.org>
Date: Thu Apr 06 18:01:06 2017

Adds button to VRShell menu mode, which lets the user manually exit WebVR presentation.
- Adds exit presentation button to the UI, which is shown in place of the back button when in WebVR presentation mode.
- Clicking the button will send a command to native and native triggers presentation exit.

BUG= 644784 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2784303002
Cr-Commit-Position: refs/heads/master@{#462542}

[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrShellDelegate.java
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/app/generated_resources.grd
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/android/vr_shell/vr_shell.cc
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/android/vr_shell/vr_shell.h
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/android/vr_shell/vr_shell_delegate.cc
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/android/vr_shell/vr_shell_delegate.h
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/resources/vr_shell/vr_shell_ui.css
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/resources/vr_shell/vr_shell_ui.html
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/resources/vr_shell/vr_shell_ui.js
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
[modify] https://crrev.com/5ca8b58d19df1aeb5d3f9e746a5d4b3f862e1087/chrome/browser/ui/webui/vr_shell/vr_shell_ui_ui.cc