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

chrome/browser/ui/webui/vr_shell/vr_shell_ui_ui.cc

Index: chrome/browser/ui/webui/vr_shell/vr_shell_ui_ui.cc
diff --git a/chrome/browser/ui/webui/vr_shell/vr_shell_ui_ui.cc b/chrome/browser/ui/webui/vr_shell/vr_shell_ui_ui.cc
index b8ed815ceb8e8f2fbe8071230714ea2868904b01..9767389fd8bd73b726930cb4b6d49825f7c29b2c 100644
--- a/chrome/browser/ui/webui/vr_shell/vr_shell_ui_ui.cc
+++ b/chrome/browser/ui/webui/vr_shell/vr_shell_ui_ui.cc
@@ -201,6 +201,8 @@ content::WebUIDataSource* CreateVrShellUIHTMLSource() {
   source->AddLocalizedString("back", IDS_VR_SHELL_UI_BACK_BUTTON);
   source->AddLocalizedString("forward", IDS_VR_SHELL_UI_FORWARD_BUTTON);
   source->AddLocalizedString("reload", IDS_VR_SHELL_UI_RELOAD_BUTTON);
+  source->AddLocalizedString("exitPresent",
+                             IDS_VR_SHELL_UI_EXIT_PRESENT_BUTTON);
   source->AddLocalizedString("newTab", IDS_VR_SHELL_NEW_TAB_BUTTON);
   source->AddLocalizedString("newIncognitoTab",
                              IDS_VR_SHELL_NEW_INCOGNITO_TAB_BUTTON);