Upgrade libwebm to libwebm-1.0.0.31 am: 76291c2912 am: 044e4840ae am: 44ea998367

Original change: https://android-review.googlesource.com/c/platform/external/libwebm/+/2812814

Change-Id: Id76d6938f622e4f424ee8d74a2128751862db2f6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>