Fix AudioMixer histogram test

If the tests are run in a different order, the test might fail.
We fix this by resetting the histogram data at the start of the test.

Change-Id: I6fb349609842b55f416cf2ec8cd93d0b4328960e
Bug: chromium:1430806
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/323801
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Auto-Submit: Fredrik Hernqvist <fhernqvist@google.com>
Cr-Commit-Position: refs/heads/main@{#40946}

modules/audio_mixer/audio_mixer_impl_unittest.cc

diff --git a/modules/audio_mixer/audio_mixer_impl_unittest.cc b/modules/audio_mixer/audio_mixer_impl_unittest.cc
index 641c966..2044cb9 100644
--- a/modules/audio_mixer/audio_mixer_impl_unittest.cc
+++ b/modules/audio_mixer/audio_mixer_impl_unittest.cc
@@ -142,6 +142,7 @@
 }
 
 TEST(AudioMixer, UpdatesSourceCountHistogram) {
+  metrics::Reset();
   constexpr int kAudioSourcesGroup1 = 5;
   constexpr int kAudioSourcesGroup2 = 3;