Move enable_webrtc to a buildflag header.

content/public/common/BUILD.gn

Index: content/public/common/BUILD.gn
diff --git a/content/public/common/BUILD.gn b/content/public/common/BUILD.gn
index fc651261001aa9f252e55b75f0a6e3053dfeb613..da1f7e972b6c2f90b2847f74c57a0388a728ea8a 100644
--- a/content/public/common/BUILD.gn
+++ b/content/public/common/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/buildflag_header.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/features.gni")
+import("//media/media_options.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//ppapi/features/features.gni")
 import("//third_party/webrtc/build/webrtc.gni")
@@ -68,6 +69,11 @@ source_set("static_switches") {
   ]
 
   public_configs = [ ":static_switches_defines" ]
+
+  # Deps required by the above headers.
+  deps = [
+    "//media:media_features",
+  ]
 }
 
 # This target allows you to use the content_features constants and statically