The idea is to allow new features/field trials to be easily added as parameters to AutofillInteractiveTest, so that tests will run all valid combinations.

Caveat that I encountered while working on https://chromium-review.googlesource.com/c/chromium/src/+/1005888/4 (patch #4): There is an interaction between AutofillInteractiveTest.CrossSitePaymentForms and field trials forced on the command line, that makes the test hang. Discussing with rogerm@, we decided to land the version in patch #2, and investigate it afterwards. The reason for the decision is that having those tests running on bots was higher priority and the code had already been approved by owners.