Pause drawing webvr when the App button is pressed

chrome/browser/android/vr_shell/BUILD.gn

Index: chrome/browser/android/vr_shell/BUILD.gn
diff --git a/chrome/browser/android/vr_shell/BUILD.gn b/chrome/browser/android/vr_shell/BUILD.gn
index 9281e2959fa9e994f32cd41081af1ff385b82624..01c66663986982020556006cbb5448c7ebc1cf14 100644
--- a/chrome/browser/android/vr_shell/BUILD.gn
+++ b/chrome/browser/android/vr_shell/BUILD.gn
@@ -52,6 +52,7 @@ if (current_cpu == "arm" || current_cpu == "arm64") {
       "ui_scene.h",
       "ui_scene_manager.cc",
       "ui_scene_manager.h",
+      "vr_browser_interface.h",
       "vr_compositor.cc",
       "vr_compositor.h",
       "vr_controller.cc",
@@ -124,6 +125,7 @@ if (enable_vr) {
       "test/paths.cc",
       "test/paths.h",
       "ui_elements/ui_element_unittest.cc",
+      "ui_scene_manager_unittest.cc",
       "ui_scene_unittest.cc",
     ]
 
@@ -132,6 +134,7 @@ if (enable_vr) {
       "//base/test:run_all_unittests",
       "//base/test:test_support",
       "//chrome/browser",
+      "//testing/gmock",
       "//testing/gtest",
       "//third_party/WebKit/public:blink",
       "//ui/gfx/geometry",