Fix sign-compare warnings on win_clang

that appear after clang roll at https://webrtc-review.googlesource.com/35741

Bug: None
Change-Id: I31193491f167e21277b9266b4331ea9212fddcbe
Reviewed-on: https://webrtc-review.googlesource.com/35783
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21421}
diff --git a/modules/audio_coding/acm2/acm_receive_test.h b/modules/audio_coding/acm2/acm_receive_test.h
index 657a124..c7e7da6 100644
--- a/modules/audio_coding/acm2/acm_receive_test.h
+++ b/modules/audio_coding/acm2/acm_receive_test.h
@@ -11,6 +11,7 @@
 #ifndef MODULES_AUDIO_CODING_ACM2_ACM_RECEIVE_TEST_H_
 #define MODULES_AUDIO_CODING_ACM2_ACM_RECEIVE_TEST_H_
 
+#include <stddef.h> // for size_t
 #include <memory>
 #include <string>
 
@@ -30,7 +31,7 @@
 
 class AcmReceiveTestOldApi {
  public:
-  enum NumOutputChannels {
+  enum NumOutputChannels : size_t {
     kArbitraryChannels = 0,
     kMonoOutput = 1,
     kStereoOutput = 2