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/LookupCacheV4.cpp

@@ -282,7 +282,7 @@ LookupCacheV4::ApplyUpdate(RefPtr<TableUpdateV4> aTableUpdate,
   // remove from the old prefix set(according to lexigraphic order).
   // |removalIndex| is the current index of RemovalIndiceArray.
   // |numOldPrefixPicked| is used to record how many prefixes we picked from the old map.
-  TableUpdateV4::RemovalIndiceArray& removalArray = aTableUpdate->RemovalIndices();
+  const TableUpdateV4::RemovalIndiceArray& removalArray = aTableUpdate->RemovalIndices();
   uint32_t removalIndex = 0;
   int32_t numOldPrefixPicked = -1;
 
@@ -525,7 +525,7 @@ ReadValue(nsIInputStream* aInputStream, T& aValue)
 ////////////////////////////////////////////////////////////////////////
 
 nsresult
-LookupCacheV4::WriteMetadata(TableUpdateV4* aTableUpdate)
+LookupCacheV4::WriteMetadata(RefPtr<const TableUpdateV4> aTableUpdate)
 {
   NS_ENSURE_ARG_POINTER(aTableUpdate);
   if (nsUrlClassifierDBService::ShutdownHasStarted()) {