discardable_memory: Add penghuang@ as owner

BUG=
TEST=

Change-Id: I30e2f477136d3ed79577368874ade5f932906a5f
Reviewed-on: https://chromium-review.googlesource.com/1120353Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571529}

components/discardable_memory/OWNERS

 reveman@chromium.org
+penghuang@chromium.org