WebView: rename SafeBrowsing tests to be more consistent

No logic change.

This CL renames safe browsing tests to a more useful naming scheme. For
each scenario, we need to test a malicious main frame and malicious
subresource separately. This naming convention makes that more explicit.

Also, this adds a message to one of the asserts that was missing one.

Review-Url: https://codereview.chromium.org/2762843002
Cr-Commit-Position: refs/heads/master@{#458275}
1 file changed