docs: add docs for fakeAsync test with custom macroTask in aio (#21669)

PR Close #21669

aio/content/examples/testing/src/app/shared/canvas.component.ts

@@ -0,0 +1,25 @@
+import { Component, AfterViewInit, ViewChild } from '@angular/core';
+
+@Component({
+  selector: 'sample-canvas',
+  template: '<canvas #sampleCanvas width="200" height="200"></canvas>'
+})
+export class CanvasComponent implements AfterViewInit {
+  blobSize: number;
+  @ViewChild('sampleCanvas') sampleCanvas;
+
+  constructor() { }
+
+  ngAfterViewInit() {
+    const canvas = this.sampleCanvas.nativeElement;
+    const context = canvas.getContext('2d');
+    if (context) {
+      context.clearRect(0, 0, 200, 200);
+      context.fillStyle = '#FF1122';
+      context.fillRect(0, 0, 200, 200);
+      canvas.toBlob((blob: any) => {
+        this.blobSize = blob.size;
+      });
+    }
+  }
+}