chromecast: Fix warnings [unused-function]

../../chromecast/media/cma/backend/volume_control.cc:57:7: error: unused function 'DbFsToScale' [-Werror,-Wunused-function]
float DbFsToScale(float db) {
      ^
1 error generated.

Bug: b/152723969
Test: Cast CQ
Change-Id: Ie8553fbf2c5b537d8e493d652e442bb6199030a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2130772Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755093}

chromecast/media/cma/backend/volume_control.cc

@@ -54,12 +54,14 @@ constexpr char kKeyAlarmDbFS[] = "dbfs.alarm";
 constexpr char kKeyCommunicationDbFS[] = "dbfs.communication";
 constexpr char kKeyDefaultVolume[] = "default_volume";
 
+#if !BUILDFLAG(SYSTEM_OWNS_VOLUME)
 float DbFsToScale(float db) {
   if (db <= kMinDbFS) {
     return 0.0f;
   }
   return std::pow(10, db / 20);
 }
+#endif
 
 std::string ContentTypeToDbFSKey(AudioContentType type) {
   switch (type) {