Adds button to VRShell menu mode, which lets the user manually exit WebVR presentation.

chrome/browser/android/vr_shell/vr_shell_delegate.cc

Index: chrome/browser/android/vr_shell/vr_shell_delegate.cc
diff --git a/chrome/browser/android/vr_shell/vr_shell_delegate.cc b/chrome/browser/android/vr_shell/vr_shell_delegate.cc
index a08b13c74bacdc25ef8809f034901a4aa3e3aff0..5b1945c6cea4e62660b4e6af680b282409109969 100644
--- a/chrome/browser/android/vr_shell/vr_shell_delegate.cc
+++ b/chrome/browser/android/vr_shell/vr_shell_delegate.cc
@@ -164,7 +164,9 @@ void VrShellDelegate::ExitWebVRPresent() {
   // VRShell is no longer needed by WebVR, allow it to shut down if it's not
   // being used elsewhere.
   JNIEnv* env = AttachCurrentThread();
-  Java_VrShellDelegate_exitWebVR(env, j_vr_shell_delegate_.obj());
+  if (Java_VrShellDelegate_exitWebVRPresent(env, j_vr_shell_delegate_.obj())) {
+    device_provider_->Device()->OnExitPresent();
+  }
 }
 
 void VrShellDelegate::CreateNonPresentingDelegate() {