App button click exits WebVR presentation.

chrome/browser/android/vr_shell/vr_browser_interface.h

Index: chrome/browser/android/vr_shell/vr_browser_interface.h
diff --git a/chrome/browser/android/vr_shell/vr_browser_interface.h b/chrome/browser/android/vr_shell/vr_browser_interface.h
index 170fe800e37fffbb20dc2cca997d436b6ee87077..3e00329e42fa5fe580f76f2f337a5e1b7cbe6c93 100644
--- a/chrome/browser/android/vr_shell/vr_browser_interface.h
+++ b/chrome/browser/android/vr_shell/vr_browser_interface.h
@@ -25,20 +25,21 @@ namespace vr_shell {
 // interface are proxies to methods on VrShell.
 class VrBrowserInterface {
  public:
-  virtual ~VrBrowserInterface() {}
+  virtual ~VrBrowserInterface() = default;
 
-  virtual void ContentSurfaceChanged(jobject surface) {}
-  virtual void GvrDelegateReady() {}
-  virtual void UpdateGamepadData(device::GvrGamepadData) {}
-  virtual void AppButtonGesturePerformed(UiInterface::Direction direction) {}
-  virtual void OnAppButtonClicked() {}
+  virtual void ContentSurfaceChanged(jobject surface) = 0;
+  virtual void GvrDelegateReady() = 0;
+  virtual void UpdateGamepadData(device::GvrGamepadData) = 0;
+  virtual void AppButtonGesturePerformed(UiInterface::Direction direction) = 0;
+  virtual void OnAppButtonClicked() = 0;
   virtual void ProcessContentGesture(
-      std::unique_ptr<blink::WebInputEvent> event) {}
-  virtual void ForceExitVr() {}
+      std::unique_ptr<blink::WebInputEvent> event) = 0;
+  virtual void ForceExitVr() = 0;
+  virtual void ExitPresent() = 0;
   virtual void RunVRDisplayInfoCallback(
       const base::Callback<void(device::mojom::VRDisplayInfoPtr)>& callback,
-      device::mojom::VRDisplayInfoPtr* info) {}
-  virtual void OnContentPaused(bool enabled) {}
+      device::mojom::VRDisplayInfoPtr* info) = 0;
+  virtual void OnContentPaused(bool enabled) = 0;
 };
 
 }  // namespace vr_shell