Always compile VP9 source files.

Instead of optionally compile VP9 source files based on the value of
the GN argument 'rtc_libvpx_build_vp9', this CL uses the preprocessor
macro RTC_ENABLE_VP9 to decide if VP9 related code needs to be compiled
or not.

Bug: None
Change-Id: I5c1b69d7ec35e8446181d98c912277d0ae8fdba2
Reviewed-on: https://webrtc-review.googlesource.com/c/111063
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25685}
diff --git a/modules/video_coding/BUILD.gn b/modules/video_coding/BUILD.gn
index 4da712b..ac359bc 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -501,19 +501,14 @@
 rtc_static_library("webrtc_vp9") {
   visibility = [ "*" ]
   poisonous = [ "software_video_codecs" ]
-  if (rtc_libvpx_build_vp9) {
-    sources = [
-      "codecs/vp9/include/vp9.h",
-      "codecs/vp9/vp9_frame_buffer_pool.cc",
-      "codecs/vp9/vp9_frame_buffer_pool.h",
-      "codecs/vp9/vp9_impl.cc",
-      "codecs/vp9/vp9_impl.h",
-    ]
-  } else {
-    sources = [
-      "codecs/vp9/vp9_noop.cc",
-    ]
-  }
+  sources = [
+    "codecs/vp9/include/vp9.h",
+    "codecs/vp9/vp9.cc",
+    "codecs/vp9/vp9_frame_buffer_pool.cc",
+    "codecs/vp9/vp9_frame_buffer_pool.h",
+    "codecs/vp9/vp9_impl.cc",
+    "codecs/vp9/vp9_impl.h",
+  ]
 
   if (!build_with_chromium && is_clang) {
     # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).