Revert 271602 "Implementation of leveldb-backed PrefStore."

Revert 271602 "Implementation of leveldb-backed PrefStore."

Caused failure:

LevelDBPrefStoreTest.GetMutableValue (run #1):
[ RUN      ] LevelDBPrefStoreTest.GetMutableValue
Received signal 11 SEGV_MAPERR 000000000008
#0 0x00000345595e base::debug::StackTrace::StackTrace()
#1 0x000003455b78 base::debug::(anonymous namespace)::StackDumpSignalHandler()
#2 0x7f22d6faccb0 \u003Cunknown>
#3 0x000003473e6b base::JSONWriter::BuildJSONString()
#4 0x000003474aca base::JSONWriter::WriteWithOptions()
#5 0x0000028861d1 Serialize()
#6 0x000002888b6e LevelDBPrefStore::ReportValueChanged()
#7 0x0000016af06d LevelDBPrefStoreTest_GetMutableValue_Test::TestBody()

http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/43483/steps/unit_tests/logs/GetMutableValue

> Implementation of leveldb-backed PrefStore.
> 
> This is not hooked up yet, migration code from Json-backed stores is needed, among other things.
> 
> BUG=362814
> 
> Review URL: https://codereview.chromium.org/169323003

TBR=dgrogan@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=271611

[Alexei Svitkine (slow), 2014-05-20 08:19:02]

Committed patchset #1 manually as r271611 (tree was closed).