Turn on lazy serialization for leveldb service.

This service is currently used only within the same process, and
potentially large amounts of data are transfered over its interfaces, so
this seems a prime candidate for lazy serialization, and should help
some with mojo localstorage performance.

Bug: 730475
Change-Id: Iff573b33f1bfda2193d6bfb5366b9d24f8190150
Reviewed-on: https://chromium-review.googlesource.com/580509Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488459}

components/leveldb/public/interfaces/BUILD.gn

@@ -5,6 +5,8 @@
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
+  support_lazy_serialization = true
+
   sources = [
     "leveldb.mojom",
   ]