[views] Fix NativeViewHost crash

NativeViewHost does not safely handle the case where it is added and
then removed from a view hierarchy without ever being added to a
Widget. Instead it crashes. This CL fixes the crash.

Bug: None
Change-Id: Ib25f754386b6512973861c4603c1f1a91a0cdb1a
Reviewed-on: https://chromium-review.googlesource.com/c/1277687
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599651}

ui/views/controls/native/native_view_host.cc

@@ -176,7 +176,7 @@ void NativeViewHost::ViewHierarchyChanged(
     if (!native_wrapper_.get())
       native_wrapper_.reset(NativeViewHostWrapper::CreateWrapper(this));
     native_wrapper_->AddedToWidget();
-  } else if (!details.is_add) {
+  } else if (!details.is_add && native_wrapper_) {
     native_wrapper_->RemovedFromWidget();
   }
 }