Improve testing of SRTP external auth code paths.

webrtc/pc/BUILD.gn

Index: webrtc/pc/BUILD.gn
diff --git a/webrtc/pc/BUILD.gn b/webrtc/pc/BUILD.gn
index eed2ee9049f4e2e1bb2fe6d32ad4121ecb2d3b8f..f84622b2314fc1c6382fe73d82fc7fa253383a76 100644
--- a/webrtc/pc/BUILD.gn
+++ b/webrtc/pc/BUILD.gn
@@ -38,6 +38,8 @@ rtc_static_library("rtc_pc") {
     "channelmanager.h",
     "currentspeakermonitor.cc",
     "currentspeakermonitor.h",
+    "externalhmac.cc",
+    "externalhmac.h",
     "mediamonitor.cc",
     "mediamonitor.h",
     "mediasession.cc",
@@ -55,12 +57,6 @@ rtc_static_library("rtc_pc") {
     "../media",
   ]
 
-  if (rtc_enable_external_auth) {
-    sources += [
-      "externalhmac.cc",
-      "externalhmac.h",
-    ]
-  }
   if (rtc_build_libsrtp) {
     deps += [ "//third_party/libsrtp" ]
   }