Re-order local destruction in ChannelMixingMatrixTest

Avoid spurious danling pointer warnings by ensuring that the matrix
(vector of floats) outlives the matrix_builder that has an unowned
reference back to it.

-- clang-format as required.

Bug: 1401495
Change-Id: I3175c4ffff9454071254ad54680a4eadaa67ec2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4350654


Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118889}