SSL-related cleanup in BrowserFrame and SslCertLookupTable

- Fix a comment in BrowserFrame.certificate()
- Simplify SslCertLookupTable by not storing 'deny' decisions.
  We only need to store 'allow' decisions, as we don't re-use 'deny' decisions.

No change in behaviour.

Bug: 5409251
Change-Id: I447cd1966fbb6c2dea8088b2e4c4e2de22405cb9