[Android WebView] Fix WebviewLayoutTest

Periodic Background Sync was disabled for WebView here:
https://chromium-review.googlesource.com/c/chromium/src/+/2419579

The layout test needs to be updated, which this CL does.

Bug: 1131660
Change-Id: Ib10f7c80ee554843c0c4a61693afeaa99a87e3d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2442730
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813203}

android_webview/tools/system_webview_shell/test/data/webexposed/not-webview-exposed.txt

@@ -134,6 +134,11 @@ interface BackgroundFetchRegistration : EventTarget
 interface ServiceWorkerRegistration : EventTarget
     getter backgroundFetch
 
+# Periodic Background Sync API is not enabled in Android webview, crbug.com/1134599
+interface PeriodicSyncManager
+interface ServiceWorkerRegistration : EventTarget
+    getter periodicSync
+
 # support_webgl2_compute_context not supported on android, crbug.com/865569
 interface WebGL2ComputeRenderingContext