[Extensions] Only require the "persistent" key to test SW extensions.

Creating a SW-based extension for testing using the
"kFlagRunAsServiceWorkerBasedExtension" flag currently requires
the "persistent" manifest key be false. This CL relaxes that
requirement so that the key only need be present.

Bug: 1093066
Change-Id: I93a2bdc2c8609e0c66f709cae11b7aff93cdb1fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316686Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791559}

chrome/browser/extensions/extension_browsertest.cc

@@ -330,9 +330,10 @@ bool ExtensionBrowserTest::CreateServiceWorkerBasedExtension(
   {
     base::Value* background_persistent = background_dict->FindKeyOfType(
         "persistent", base::Value::Type::BOOLEAN);
-    if (!background_persistent || background_persistent->GetBool()) {
+    if (!background_persistent) {
       ADD_FAILURE() << path.value()
-                    << ": Only event pages can be loaded as SW extension.";
+                    << ": The \"persistent\" key must be specified to run as a "
+                       "Service Worker-based extension.";
       return false;
     }
   }