Load CMA backend from shared library

chromecast/media/cma/backend/BUILD.gn

Index: chromecast/media/cma/backend/BUILD.gn
diff --git a/chromecast/media/cma/backend/BUILD.gn b/chromecast/media/cma/backend/BUILD.gn
index 51e62441010c34acbdc496c65c0efb34f1b8ac56..0f1dc55ed3f49a65c9937af31829fb5a8a60d717 100644
--- a/chromecast/media/cma/backend/BUILD.gn
+++ b/chromecast/media/cma/backend/BUILD.gn
@@ -4,28 +4,14 @@
 
 source_set("backend") {
   sources = [
-    "audio_pipeline_device.cc",
-    "audio_pipeline_device.h",
     "audio_pipeline_device_default.cc",
     "audio_pipeline_device_default.h",
-    "media_clock_device.cc",
-    "media_clock_device.h",
     "media_clock_device_default.cc",
     "media_clock_device_default.h",
-    "media_component_device.cc",
-    "media_component_device.h",
     "media_component_device_default.cc",
     "media_component_device_default.h",
-    "media_pipeline_device.cc",
-    "media_pipeline_device.h",
-    "media_pipeline_device_factory.h",
-    "media_pipeline_device_factory_default.cc",
-    "media_pipeline_device_factory_default.h",
-    "media_pipeline_device_factory_simple.cc",
-    "media_pipeline_device_params.cc",
-    "media_pipeline_device_params.h",
-    "video_pipeline_device.cc",
-    "video_pipeline_device.h",
+    "media_pipeline_backend_default.cc",
+    "media_pipeline_backend_default.h",
     "video_pipeline_device_default.cc",
     "video_pipeline_device_default.h",
   ]