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

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

Change-Id: I8a1de754051ecbcd9845e03cdcaee0c7a34cbc28
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>