Update external/libvpx to v1.6.1

Bug: 33579953
Test: build.webmproject.org automated tests, chromium tests, manual
builds.

Change-Id: I87e9a4e6a0c55465ecbf54cca64ccc24a9f4b57c
Merged-In: I87e9a4e6a0c55465ecbf54cca64ccc24a9f4b57c
(cherry picked from commit 7bc9febe8749e98a3812a0dc4380ceae75c29450)
919 files changed