Revert 272671 "Implementation of leveldb-backed PrefStore."

Revert 272671 "Implementation of leveldb-backed PrefStore."

The CL broke asan/lsan bots on linux, and chromiumos e.g.

LevelDBPrefStoreTest.BasicObserver (run #1):
Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x5babfb in operator new(unsigned long) /usr/local/google/work/chromium/src/third_party/llvm/projects/compiler-rt/lib/asan/asan_new_delete.cc:55
    #1 0x61f3699 in LevelDBPrefStore::OnStorageRead(scoped_ptr\u003CLevelDBPrefStore::ReadingResults, base::DefaultDeleter\u003CLevelDBPrefStore::ReadingResults> >) chrome/browser/prefs/leveldb_pref_store.cc:406
    #2 0x61f2ea7 in LevelDBPrefStore::ReadPrefs() chrome/browser/prefs/leveldb_pref_store.cc:329
    #3 0x336efee in LevelDBPrefStoreTest_BasicObserver_Test::TestBody() chrome/browser/prefs/leveldb_pref_store_unittest.cc:97
...

Indirect leak of 4096 byte(s) in 1 object(s) allocated from:
    #0 0x5bad4b in operator new[](unsigned long) /usr/local/google/work/chromium/src/third_party/llvm/projects/compiler-rt/lib/asan/asan_new_delete.cc:57
    #1 0x7e66db5 in AllocateNewBlock third_party/leveldatabase/src/util/arena.cc:62
    #2 0x7e66db5 in leveldb::Arena::AllocateFallback(unsigned long) third_party/leveldatabase/src/util/arena.cc:33
    #3 0x7e2a88a in NewNode third_party/leveldatabase/src/db/skiplist.h:182
    #4 0x7e2a88a in leveldb::SkipList\u003Cchar const*, leveldb::MemTable::KeyComparator>::SkipList(leveldb::MemTable::KeyComparator, leveldb::Arena*) third_party/leveldatabase/src/db/skiplist.h:325
    #5 0x7e2a76e in leveldb::MemTable::MemTable(leveldb::InternalKeyComparator const&) third_party/leveldatabase/src/db/memtable.cc:24
    #6 0x7e084ce in leveldb::DBImpl::DBImpl(leveldb::Options const&, std::string const&) third_party/leveldatabase/src/db/db_impl.cc:120
    #7 0x7e1bfb0 in leveldb::DB::Open(leveldb::Options const&, std::string const&, leveldb::DB**) third_party/leveldatabase/src/db/db_impl.cc:1449
    #8 0x61ef0a2 in LevelDBPrefStore::OpenDB(base::FilePath const&, LevelDBPrefStore::ReadingResults*) chrome/browser/prefs/leveldb_pref_store.cc:113
    #9 0x61efcae in LevelDBPrefStore::DoReading(base::FilePath const&) chrome/browser/prefs/leveldb_pref_store.cc:158
    #10 0x61f2dc0 in LevelDBPrefStore::ReadPrefs() chrome/browser/prefs/leveldb_pref_store.cc:325
...

> Implementation of leveldb-backed PrefStore.
>
> This is not hooked up yet, migration code from Json-backed stores is needed, among other things.
>
> BUG=362814
>
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=271602
>
> Review URL: https://codereview.chromium.org/169323003

TBR=dgrogan@chromium.org

Review URL: https://codereview.chromium.org/298043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272700 0039d316-1c4b-4281-b951-d872f2087c98