Add GN Build file for extensions_common target.

content/renderer/BUILD.gn

Index: content/renderer/BUILD.gn
diff --git a/content/renderer/BUILD.gn b/content/renderer/BUILD.gn
index 847c75283abaf16ab55702e741947d6686ce7d6b..1b83c38ceabfcbff6463400d7df47a781f632d43 100644
--- a/content/renderer/BUILD.gn
+++ b/content/renderer/BUILD.gn
@@ -74,7 +74,6 @@ source_set("renderer") {
     sources -= [
       "accessibility/renderer_accessibility_focus_only.cc",
       "media/audio_decoder.cc",
-      "media/filter_helpers.cc",
       "media/webmediaplayer_impl.cc",
     ]
     sources += [
@@ -169,11 +168,4 @@ source_set("renderer") {
         content_renderer_gypi_values.private_renderer_plugin_webrtc_sources,
         ".", "//content")
   }
-
-  if (!enable_pepper_cdms) {
-    sources -= [
-      "renderer/media/crypto/ppapi_decryptor.cc",
-      "renderer/media/crypto/ppapi_decryptor.h",
-    ]
-  }
 }