system-web-apps: re-enable SystemWebAppManagerUpgrade test

This test was disabled due to https://crrev.com/c/2602041 triggering a
DCHECK in blink.

https://crrev.com/c/2612357 fixed the CSS file triggering the DCHECK,
thus re-enable the test.

Fixed: 1162044
Change-Id: I83f3feea04c6092820f15160fc063a7566e4e701
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623607Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Jiewei Qian  <qjw@chromium.org>
Auto-Submit: Jiewei Qian  <qjw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843223}

chrome/browser/web_applications/system_web_app_manager_browsertest.cc

@@ -1126,9 +1126,7 @@ IN_PROC_BROWSER_TEST_P(SystemWebAppManagerUpgradeBrowserTest, PRE_Upgrade) {
             GetManager().GetAppIds().size());
 }
 
-// Test is consistently failing: crbug/1162044
-IN_PROC_BROWSER_TEST_P(SystemWebAppManagerUpgradeBrowserTest,
-                       DISABLED_Upgrade) {
+IN_PROC_BROWSER_TEST_P(SystemWebAppManagerUpgradeBrowserTest, Upgrade) {
   WaitForSystemAppsSynchronized();
   const auto& app_ids = GetManager().GetAppIds();