Make Safe Browsing Subresource Filter tests truly end-to-end.
As part of deprecating ContentSubresourceFilterDriver, remove a usage in
SafeBrowsingServiceTest, where a mock version of the driver is used to check
whether the subresource filtering activation IPC message is sent after the user
clicks through an interstitial.
Instead, use this opportunity to make this a truly end-to-end test, which
checks activation by probing whether an artificially disallowed resource indeed
fails to load. Also extend the test with verification that filtering is again
activated when the page is navigated to a second time when the interstitial is
not shown again.