Allow for Instanbul coverage with Playwright

This makes the PlaywrightLauncherPage function similarly to the
ChromeLauncher where we attempt to load the coverage from the browser
and return that if availble during the stopSession method. Currently the
Playwright launcher breaks early if the native instrumentation is not
enabled.

We changed to using the Playwright launcher and were suprised that our
coverage stopped working even though we are using the babel istanbul
plugin which worked when we were using the ChromeLauncher.

.changeset/tidy-knives-agree.md

@@ -0,0 +1,5 @@
+---
+'@web/test-runner-playwright': patch
+---
+
+Support browser generated coverage without native instrumentation

packages/test-runner-playwright/src/PlaywrightLauncherPage.ts

@@ -44,9 +44,7 @@ export class PlaywrightLauncherPage {
   }
 
   async stopSession(): Promise<SessionResult> {
-    const testCoverage = this.nativeInstrumentationEnabledOnPage
-      ? await this.collectTestCoverage(this.config, this.testFiles)
-      : undefined;
+    const testCoverage = await this.collectTestCoverage(this.config, this.testFiles);
 
     // navigate to an empty page to kill any running code on the page, stopping timers and
     // breaking a potential endless reload loop
@@ -82,6 +80,10 @@ export class PlaywrightLauncherPage {
       );
     }
 
+    if (!this.nativeInstrumentationEnabledOnPage) {
+      return undefined;
+    }
+
     // get native coverage from playwright
     let coverage: V8Coverage[];
     if (this.product === 'chromium') {