Initial implementation of collecting stylesheet and script size.

Closes #3436527

core/profiles/demo_umami/tests/src/FunctionalJavascript/AssetAggregationAcrossPagesTest.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\demo_umami\FunctionalJavascript;
+
+use Drupal\FunctionalJavascriptTests\PerformanceTestBase;
+use Drupal\Tests\PerformanceData;
+
+/**
+ * Tests demo_umami profile performance.
+ *
+ * @group #slow
+ */
+class AssetAggregationAcrossPagesTest extends PerformanceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $profile = 'demo_umami';
+
+  /**
+   * Checks the asset requests made when the front and recipe pages are visited.
+   */
+  public function testFrontAndRecipesPages() {
+    $performance_data = $this->doRequests();
+    $this->assertSame(4, $performance_data->getStylesheetCount());
+    $this->assertSame(90241, $performance_data->getStylesheetBytes());
+    $this->assertSame(2, $performance_data->getScriptCount());
+    $this->assertSame(14150, $performance_data->getScriptBytes());
+  }
+
+  /**
+   * Checks the asset requests made when the front and recipe pages are visited.
+   */
+  public function testFrontAndRecipesPagesAuthenticated() {
+    $user = $this->createUser();
+    $this->drupalLogin($user);
+    $this->rebuildAll();
+    $performance_data = $this->doRequests();
+    $this->assertSame(4, $performance_data->getStylesheetCount());
+    $this->assertSame(94355, $performance_data->getStylesheetBytes());
+    $this->assertSame(2, $performance_data->getScriptCount());
+    $this->assertSame(264076, $performance_data->getScriptBytes());
+  }
+
+  /**
+   * Helper to do requests so the above test methods stay in sync.
+   */
+  protected function doRequests(): PerformanceData {
+    $performance_data = $this->collectPerformanceData(function () {
+      $this->drupalGet('<front>');
+      // Give additional time for the request and all assets to be returned
+      // before making the next request.
+      sleep(2);
+      $this->drupalGet('articles');
+    }, 'umamiFrontAndRecipePages');
+    return $performance_data;
+  }
+
+}