external/libvpx/libwebm: Update snapshot

Update libwebm snapshot. This update contains security fixes from upstream.

Upstream git hash: 229f49347d19b0ca0941e072b199a242ef6c5f2b

BUG=23167726

Change-Id: Id3e140e7b31ae11294724b1ecfe2e9c83b4d4207
(cherry picked from commit d0281a15b3c6bd91756e453cc9398c5ef412d99a)

const Block::Frame& Block::GetFrame(int idx) const {
  assert(idx >= 0);
  assert(idx < m_frame_count);

 const Frame& f = m_frames[idx];
  assert(f.pos > 0);
  assert(f.len > 0);

 return f;
}

const Block::Frame& Block::GetFrame(int idx) const {
  assert(idx >= 0);
  assert(idx < m_frame_count);

 const Frame& f = m_frames[idx];
  assert(f.pos > 0);
  assert(f.len > 0);

 return f;
}
