feat(content): add a resize function to recalculate the content size

this should be called when dynamically displaying/hiding headers,
footers, or tabs.

references #7131

src/components/content/content.ts

@@ -442,6 +442,45 @@ export class Content extends Ion {
     }
   }
 
+  /**
+   * Tell the content to recalculate its dimensions. This should be called
+   * after dynamically adding headers, footers, or tabs.
+   *
+   * ```ts
+   * @Component({
+   *   template: `
+   *     <ion-header>
+   *       <ion-navbar>
+   *         <ion-title>Main Navbar</ion-title>
+   *       </ion-navbar>
+   *       <ion-toolbar *ngIf="showToolbar">
+   *         <ion-title>Dynamic Toolbar</ion-title>
+   *       </ion-toolbar>
+   *     </ion-header>
+   *     <ion-content>
+   *       <button (click)="toggleToolbar()">Toggle Toolbar</button>
+   *     </ion-content>
+   * `})
+   *
+   * class E2EPage {
+   *   @ViewChild(Content) content: Content;
+   *   showToolbar: boolean = false;
+   *
+   *   toggleToolbar() {
+   *     this.showToolbar = !this.showToolbar;
+   *     this.content.resize();
+   *   }
+   * }
+   * ```
+   *
+   */
+  resize() {
+    nativeRaf(() => {
+      this.readDimensions();
+      this.writeDimensions();
+    });
+  }
+
   /**
    * @private
    * DOM READ