Build a few more extensions files only when extensions are enabled.

chrome/common/extensions/features/feature_channel.cc

Index: chrome/common/extensions/features/feature_channel.cc
diff --git a/chrome/common/extensions/features/feature_channel.cc b/chrome/common/extensions/features/feature_channel.cc
index 98a178ef375f00117e7e00767029cc2218195bc1..e1cce9f61540321ca0a8a59d12603570534f699c 100644
--- a/chrome/common/extensions/features/feature_channel.cc
+++ b/chrome/common/extensions/features/feature_channel.cc
@@ -27,4 +27,14 @@ VersionInfo::Channel GetDefaultChannel() {
   return kDefaultChannel;
 }
 
+ScopedCurrentChannel::ScopedCurrentChannel(VersionInfo::Channel channel)
+    : original_channel_(VersionInfo::CHANNEL_UNKNOWN) {
+  original_channel_ = GetCurrentChannel();
+  SetCurrentChannel(channel);
+}
+
+ScopedCurrentChannel::~ScopedCurrentChannel() {
+  SetCurrentChannel(original_channel_);
+}
+
 }  // namespace extensions