WebView: remove @Nullable from SB whitelist

This removes the @Nullable annotation from
WebView#setSafeBrowsingWhitelist's List parameter. This does not need to
be Nullable, since the whitelist can be unset just as easily by passing
an empty list.

Bug: 63084528
Test: N/A
Change-Id: Ica9d67bc907b16e89d0a1b6a15d91e21f74cc4c9