Upgrade libwebm to libwebm-1.0.0.31

This project was upgraded with external_updater.
Usage: tools/external_updater/updater.sh update libwebm
For more info, check https://cs.android.com/android/platform/superproject/+/main:tools/external_updater/README.md

Bug: 308688826
Test: TreeHugger
Test: atest CtsMediaV2TestCases:ExtractorTest \
CtsMediaV2TestCases:ExtractorUnitTest \
CtsMediaV2TestCases:MuxerTest \
CtsMediaV2TestCases:MuxerUnitTest
(cherry picked from https://android-review.googlesource.com/q/commit:76291c2912ba965a35f0dbdc0103c38289df3773)
Merged-In: I18bedd39853cf9c79053614c4ceb4f54ad430956
Change-Id: I18bedd39853cf9c79053614c4ceb4f54ad430956
7 files changed