Upgrade libwebm to libwebm-1.0.0.31 am: 76291c2912

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

Change-Id: I1009189aab1b42ac35ce9478fdc2fc8158cdc655
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>