bug: ion-back-button does not display if wrapped in custom component

Bug Report
Ionic version:
[x] 4.0.0.rc-1

Current behavior:
I do see in the element list that the second page in the stack gets an can-go-back class.
However the ion-back-button remains invisible (display:none)

Expected behavior:
ion-back-button should be visible. As there is a previous page in the stack.

Steps to reproduce:
I have a page with a sidepanel menu. Which is a tab page. I navigate to the child page with navigateForward(), I can go back either by swiping back or pressing the tab icon.

Ionic info:
```
Ionic:

ionic (Ionic CLI) : 4.7.0 (/usr/local/lib/node_modules/ionic)
Ionic Framework : @ionic/angular 4.0.0-rc.1
@angular-devkit/build-angular : 0.11.4
@angular-devkit/schematics : 7.2.1
@angular/cli : 7.2.1
@ionic/angular-toolkit : 1.2.2

Cordova:

cordova (Cordova CLI) : 8.1.2 (cordova-lib@8.1.1)
Cordova Platforms : android 7.1.4, ios 4.5.5
Cordova Plugins : cordova-plugin-ionic-keyboard 2.1.3, cordova-plugin-ionic-webview 2.3.1, (and 24 other plugins)

System:

ios-deploy : 1.9.4
NodeJS : v11.6.0 (/usr/local/Cellar/node/11.6.0/bin/node)
npm : 6.5.0
OS : macOS Mojave
Xcode : Xcode 10.1 Build version 10O45e

```

Start by reproducing the issue with an ion-back-button wrapped in a custom component on a child page reached with navigateForward(), and inspect the can-go-back class and display state. Trace how the wrapped button determines whether a previous page exists. Done means the ion-back-button is visible whenever the navigation stack has a previous page, including this side-panel tab scenario.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.