Bug 1311935 - P3. Implement safebrowsing v4 caching logic. r=francois

LookupCacheV4::Has implements safebrowsing v4 caching logic.
1. Check if fullhash match any prefix in local database:
  - If not, the URL is safe.
2. Check if prefix is in the cache(prefix is always the first 4-byte of
   the fullhash, Bug 1323953):
  - If not, send fullhash request
3. Check if fullhash is in the positive cache:
  - If fullhash is found and it is not expired, the URL is not safe.
  - If fullhash is found and it is expired, send fullhash request.
4. If fullhash is not found, check negative cache expired time:
  - If negative cache time is not expired, the URL is safe.
  - If negative cache time is expired, send fullhash request.

MozReview-Commit-ID: GRX7CP8ig49

toolkit/components/url-classifier/VariableLengthPrefixSet.cpp

@@ -188,6 +188,7 @@ VariableLengthPrefixSet::Matches(const nsACString& aFullHash, uint32_t* aLength)
   for (auto iter = mVLPrefixSet.ConstIter(); !iter.Done(); iter.Next()) {
     if (BinarySearch(aFullHash, *iter.Data(), iter.Key())) {
       *aLength = iter.Key();
+      MOZ_ASSERT(*aLength > 4);
       return NS_OK;
     }
   }