When running
WebstoreInlineInstallerListenerTest.BothListenersTest
browser test using "--enable-features=NetworkService" flag, it fails at:

../../chrome/browser/extensions/webstore_installer_test.cc:113: Failure
Value of: result
  Actual: false
Expected: true

../../chrome/browser/extensions/webstore_inline_installer_browsertest.cc:550: Failure
Value of: registry->enabled_extensions().GetByID(kTestExtensionId)
  Actual: false
Expected: true

extension_service.cc(776) Check failed: extension.get().

It looks like the extension hasn't been installed when the test runs.