Extended LevelDB.ApproximateMemTableMemoryUse histogram expiration date

LevelDB.ApproximateMemTableMemoryUse helps us track memory usage for
leveldb_proto databases, as we'll migrate clients from multiple
instances to a single shared instance this histogram will be useful to
track possible performance regressions.

This CL extends the histogram's expiration by 1 year, and replaces ssid@
with salg@ as an owner.

Bug: 986506
Change-Id: I30c472c8ccc3df844de85e6890e17e2ce0dc0234
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715543
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Salvador Guerrero <salg@google.com>
Cr-Commit-Position: refs/heads/master@{#680461}