Roll libwebp to 0.6.1

Bug: oss-fuzz:6299
Change-Id: I2e87368e04ff37db7e431fd1960f0bf91440037f
Reviewed-on: https://skia-review.googlesource.com/107281
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
2 files changed