mojo: Start building a leveldb service.

This service encapsulates just leveldb and exports an interface for
Get/Put/Write. Instead of writing directly to the filesystem, it instead
takes a mojo:filesystem DirectoryPtr where it stores its data, letting
leveldb be entirely sandboxed.

BUG=585587

Review URL: https://codereview.chromium.org/1682803004

Cr-Commit-Position: refs/heads/master@{#375949}

diff --git a/BUILD.gn b/BUILD.gn
index 929c5ad..57087a37 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -596,6 +596,7 @@
   if (is_win || is_linux) {
     deps += [
       "//components/filesystem:apptests",
+      "//components/leveldb:apptests",
       "//components/mus/ws:tests",
       "//components/resource_provider:apptests",
       "//components/resource_provider:resource_provider_unittests",