[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in leveldb_proto

MessageLoop will go away, eventually.

BUG=891670

Change-Id: I8f6b7e62166de2d025f05cf4239686fed7a1d3ec
Reviewed-on: https://chromium-review.googlesource.com/c/1353884
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#615979}
1 file changed