App button click exits WebVR presentation.

chrome/browser/android/vr_shell/ui_scene_manager.cc

Index: chrome/browser/android/vr_shell/ui_scene_manager.cc
diff --git a/chrome/browser/android/vr_shell/ui_scene_manager.cc b/chrome/browser/android/vr_shell/ui_scene_manager.cc
index 5d4f55aefcfd060fa6f1a8170ba75d20015504fc..7ae408e94ae49e493ac1ab67ba79ca0c1792d6a7 100644
--- a/chrome/browser/android/vr_shell/ui_scene_manager.cc
+++ b/chrome/browser/android/vr_shell/ui_scene_manager.cc
@@ -185,11 +185,8 @@ void UiSceneManager::SetWebVRSecureOrigin(bool secure) {
 }
 
 void UiSceneManager::OnAppButtonClicked() {
-  // Pressing the app button currenly pauses content rendering. Note: its still
-  // unclear what we want to do here and this will most likely change.
-  content_rendering_enabled_ = !content_rendering_enabled_;
-  scene_->SetWebVrRenderingEnabled(content_rendering_enabled_);
-  browser_->OnContentPaused(!content_rendering_enabled_);
+  // App button click exits the WebVR presentation.
+  browser_->ExitPresent();

[cjgrant, 2017/05/08 15:29:40]

The UI should know it's presenting WebVR.  But I guess no point in checking?

[amp, 2017/05/08 17:48:13]

What happens when you press the app button in vr shell (and/or cinema mode)?

Are we going to expand this method to cover those use cases later?

[tiborg, 2017/05/08 18:59:37]

cjgrant@: I think it is better to let the delegate handle an exit request even
if we are not currently presenting. The rare case can happen that you click the
app button twice before we actually exit presentation. So the delegate must be
able to deal with an exit request while not presenting anyway.

amp@: You mean VR Browsing Mode? Currently, nothing happens in VR Browsing Mode.
Would like to hook up the app button to something for v0? Like exiting
fullscreen?

[amp, 2017/05/08 19:55:22]

Yes exiting full screen is what I had in mind.  All we need to do is send a
message back to the web contents (see line 427 of vr_shell.cc in the change).

If we renamed it to something like ExitExclusiveScreenMode (I'm sure we can do
better than that mouthful) then it could perhaps work for both cases.

If we can't get it in for v0 I think it's fine though, as cinema mode wasn't a
critical requirement, just a nice to have.

[tiborg, 2017/05/08 20:14:01]

Yeah, exiting fullscreen should be very easy. What do you think about adding an
ExitFullscreen function to VrShell and call it from here? I would do this in a
follow up CL though.

[amp, 2017/05/08 20:37:02]

Follow up cl is fine.  We will need to wire up the app button, the back button,
and a new exit button that will all do the same thing for full screen eventually
in vr shell so having a specific method for it sounds good.

 }
 
 void UiSceneManager::ConfigureSecurityWarnings() {