[chromecast] Slew stream volume changes in StreamMixerAlsa.

chromecast/media/cma/backend/alsa/BUILD.gn

Index: chromecast/media/cma/backend/alsa/BUILD.gn
diff --git a/chromecast/media/cma/backend/alsa/BUILD.gn b/chromecast/media/cma/backend/alsa/BUILD.gn
index d03c575e3753c33afdb3a67b1745e1faa318d974..54ede7e6f785ffd75841e3a188f4b4a91d4a6d6f 100644
--- a/chromecast/media/cma/backend/alsa/BUILD.gn
+++ b/chromecast/media/cma/backend/alsa/BUILD.gn
@@ -55,6 +55,7 @@ source_set("alsa_cma_backend") {
   deps = [
     ":alsa_features",
     ":audio_filter_includes",
+    ":slew_volume",
     "//base",
     "//chromecast/base",
     "//chromecast/media/cma/backend",
@@ -94,6 +95,17 @@ source_set("audio_filter_includes") {
   ]
 }
 
+source_set("slew_volume") {
+  sources = [
+    "slew_volume.cc",
+    "slew_volume.h",
+  ]
+  deps = [
+    "//base",
+    "//media",
+  ]
+}
+
 # GYP target: chromecast/media/media.gyp:chromecast_alsa_features
 buildflag_header("alsa_features") {
   header = "alsa_features.h"