New PerformanceTestBase added to core

A new Drupal\FunctionalJavascriptTests\PerformanceTestBase has been added to core. Most of the logic is in Drupal\Tests\PerformanceTestTrait

This uses different webdriver settings to enable collection of performance statistics from the browser.

The class currently keeps track of the number of script and stylesheets and in the future may be expanded to enable other performance benchmarking.

The primary API is the ::collectPerformanceData() method, basic usage:

   $performance_data = $this->collectPerformanceData(function () {
      $this->drupalGet('node/1');
    });
    $this->assertSame(2, $performance_data->getStylesheetCount());
    $this->assertSame(1, $performance_data->getScriptCount());
    $this->assertSame(92, $performance_data->getQueryCount());