Upgrade libwebm to libwebm-1.0.0.31 am: 76291c2912

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

Change-Id: Ia46a9b209b4f3e043659c9d592ba638eb5a50813
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>