vr: Make an undefined method pure virtual.

GvrDelegateProvider::OnListeningForActivateChanged() is not defined, yet
isn't marked as pure virtual. This is suspected to be the cause of the
orderfile bot failing (see the linked bug), as the error message says:
"the vtable symbol may be undefined because the class is missing its key
function", and a key function is "the first non-pure, non-inline,
virtual method".

TBR=kbr

Bug: 747342
Change-Id: I32f70a0aaac9130692f36b1976b18f0df874df2f
Reviewed-on: https://chromium-review.googlesource.com/586722
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489665}

device/vr/android/gvr/gvr_delegate_provider.h

@@ -31,7 +31,7 @@ class DEVICE_VR_EXPORT GvrDelegateProvider {
   virtual GvrDelegate* GetDelegate() = 0;
   virtual void OnDisplayAdded(VRDisplayImpl* display) = 0;
   virtual void OnDisplayRemoved(VRDisplayImpl* display) = 0;
-  virtual void OnListeningForActivateChanged(VRDisplayImpl* display);
+  virtual void OnListeningForActivateChanged(VRDisplayImpl* display) = 0;
   virtual void CreateVRDisplayInfo(
       const base::Callback<void(mojom::VRDisplayInfoPtr)>& callback,
       uint32_t device_id) = 0;