Bug 1434206 - Make TableUpdate objects const as much as possible. r=gcp

I tried to make TableUpdateArray point to const TableUpdate objects
everywhere but there were two problems:

- HashStore::ApplyUpdate() triggers a few Merge() calls which include
  sorting the underlying TableUpdate object first.

- LookupCacheV4::ApplyUpdate() calls TableUpdateV4::NewChecksum() when the
  checksum is missing and that sets mChecksum.

MozReview-Commit-ID: LIhJcoxo7e7

toolkit/components/url-classifier/HashStore.cpp

@@ -571,7 +571,7 @@ template<class T>
 static nsresult
 Merge(ChunkSet* aStoreChunks,
       FallibleTArray<T>* aStorePrefixes,
-      ChunkSet& aUpdateChunks,
+      const ChunkSet& aUpdateChunks,
       FallibleTArray<T>& aUpdatePrefixes,
       bool aAllowMerging = false)
 {