Roll src/third_party/libvpx/source/libvpx/ 18d260d13..d8f89c49e (2 commits)

https://chromium.googlesource.com/webm/libvpx.git/+log/18d260d13f2b..d8f89c49e17b

$ git log 18d260d13..d8f89c49e --date=short --no-merges --format='%ad %ae %s'
2018-12-14 jianj vp8: Fix potential use-after-free in mfqe.
2018-12-13 sdeng Remove unused code in tiny_ssim

Created with:
  roll-dep src/third_party/libvpx/source/libvpx
R=johannkoenig@google.com

BUG=913246

Change-Id: I886386eaf644c2c5c85234451c5fd53a042920a2
Reviewed-on: https://chromium-review.googlesource.com/c/1380536
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Reviewed-by: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#617232}