We're currently using the back button to get out of fullscreen mode, so whatever we implement here should also take that into account.

acondor@ is working on this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/87cc44e2eb708c276f8fb2b6644588753ed75c6a

commit 87cc44e2eb708c276f8fb2b6644588753ed75c6a
Author: acondor <acondor@google.com>
Date: Wed Mar 08 23:24:16 2017

Visually disable VR Shell back/forward buttons when no history is available

The CSS class was defined for this purpose is 'disabled-button'

BUG= 684634 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2735983004
Cr-Commit-Position: refs/heads/master@{#455591}

[modify] https://crrev.com/87cc44e2eb708c276f8fb2b6644588753ed75c6a/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrShellImpl.java
[modify] https://crrev.com/87cc44e2eb708c276f8fb2b6644588753ed75c6a/chrome/browser/android/vr_shell/ui_interface.cc
[modify] https://crrev.com/87cc44e2eb708c276f8fb2b6644588753ed75c6a/chrome/browser/android/vr_shell/ui_interface.h
[modify] https://crrev.com/87cc44e2eb708c276f8fb2b6644588753ed75c6a/chrome/browser/android/vr_shell/vr_shell.cc
[modify] https://crrev.com/87cc44e2eb708c276f8fb2b6644588753ed75c6a/chrome/browser/android/vr_shell/vr_shell.h
[modify] https://crrev.com/87cc44e2eb708c276f8fb2b6644588753ed75c6a/chrome/browser/resources/vr_shell/vr_shell_ui.css
[modify] https://crrev.com/87cc44e2eb708c276f8fb2b6644588753ed75c6a/chrome/browser/resources/vr_shell/vr_shell_ui.js
[modify] https://crrev.com/87cc44e2eb708c276f8fb2b6644588753ed75c6a/chrome/browser/resources/vr_shell/vr_shell_ui_api.js