leveldb: Disable read caches for in-memory databases.

In-memory databases are already fast and a read cache will probably
have little or no benefit. Disabling the cache to conserve RAM.

This change is a subset of http://crrev.com/c/647329 (reverted) without the
changes related to the creation of leveldb_chrome::NewMemEnv(). That was
reverted due to a bug in leveldb which was fixed in
https://github.com/google/leveldb/commit/1c75e88055e06.

Bug: 761398
Change-Id: I64dbf53dbf74ec251afd84aec9b9d751508054ec
Reviewed-on: https://chromium-review.googlesource.com/704034
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507503}

• third_party/leveldatabase/README.chromium[diff]
• third_party/leveldatabase/env_chromium.cc[diff]
• third_party/leveldatabase/env_chromium.h[diff]
• third_party/leveldatabase/env_chromium_unittest.cc[diff]
• third_party/leveldatabase/leveldb_chrome.cc[diff]
• third_party/leveldatabase/leveldb_chrome.h[diff]