Revert "update libwebm"

Revert "update libwebm"

This reverts commit 595edb9669e27927726523c3f463d0eb6ea73f30.

Incorrect check_add_cxxflags invocation prevented libwebm from
building. Correcting it causes build failures on jenkins and mac.

Original change's description:
> update libwebm
>
> Clears "auto_ptr deprecated" warnings when building with
> clang v6.0.0
>
> Requires C++11 support.
>
> Change-Id: I5ea2744e73deeaa4e7b2599bacf0b6c9cf355a54

TBR=jzern@google.com,johannkoenig@google.com,builds@webmproject.org

Change-Id: I7340d912a121de035997cbd8ad77a150ee38189a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true