IndexedDB: add non-modifying methods to check index existance

• Component: WebCore Misc.
• Status: RESOLVED WONTFIX

[Alec Flett]

The current findKeyInIndex (in IDBLevelDBBackingStore.cpp) sometimes modifies the database to remove stale index entries. Sometimes this is fine, but sometimes when you're looking up a index key multiple times (like once to check if it's there, and a second time to find which record to overwrite) you want to have a little more control over when we're actually modifying the database. More concretely, this means that IDBBackingStore::keyExistsInIndex() and IDBBackingStore::getPrimaryKeyViaIndex cannot be const, which is going to possibly complicate caching down the road in IDB.Next.

So what we need is two version of findKeyInIndex, and figure out when is a good time to call the const vs the non-const one.