Move enable_webrtc to a buildflag header.

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index 371f0ee2a5742d658ad00abceeb5e574028975c7..1668c5a7816c9fd57bad58df9277e38d3a04fefc 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -4,6 +4,7 @@
 
 #include "build/build_config.h"
 #include "content/public/common/content_switches.h"
+#include "media/media_features.h"
 
 namespace switches {
 
@@ -896,7 +897,7 @@ const char kZygoteCmdPrefix[]               = "zygote-cmd-prefix";
 // Causes the process to run as a renderer zygote.
 const char kZygoteProcess[]                 = "zygote";
 
-#if defined(ENABLE_WEBRTC)
+#if BUILDFLAG(ENABLE_WEBRTC)
 // Disables HW decode acceleration for WebRTC.
 const char kDisableWebRtcHWDecoding[]       = "disable-webrtc-hw-decoding";