Upgrade libwebm to libwebm-1.0.0.31 am: 76291c2912 am: 1eadf6acb3 am: 0cb89e3f3a

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

Change-Id: I896ba4156a1d161ac92a293f5491da704ab9a28c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>