Log when webview requests credential

Log when webview requests credential from credential manager. Credential
Autofill Service determines that a credential response is flagged as
webview requested when the assistStrcture has a viewnode that has both
webdomain and credential option.

Bug: 322036652
Test: built and deployed locally. Checked the log of the metric with
formgen and addressbook

Change-Id: I496c82eb98aacb87fbf2eef0753ce83331e4846e