Move enable extensions define to a build flag.

chrome/browser/io_thread.cc

Index: chrome/browser/io_thread.cc
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index 48a61fcad2e32007cb4012e9cca19c35849d0007..f25a7ec55cfa5ae224b93b1dce1cf8ccb7677b5e 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -65,6 +65,7 @@
 #include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/user_agent.h"
+#include "extensions/features/features.h"
 #include "net/base/host_mapping_rules.h"
 #include "net/base/logging_network_change_observer.h"
 #include "net/base/sdch_manager.h"
@@ -109,7 +110,7 @@
 #include "net/url_request/url_request_job_factory_impl.h"
 #include "url/url_constants.h"
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 #include "chrome/browser/extensions/event_router_forwarder.h"
 #endif
 
@@ -317,7 +318,7 @@ IOThread::IOThread(
     net_log::ChromeNetLog* net_log,
     extensions::EventRouterForwarder* extension_event_router_forwarder)
     : net_log_(net_log),
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
       extension_event_router_forwarder_(extension_event_router_forwarder),
 #endif
       globals_(nullptr),
@@ -494,7 +495,7 @@ void IOThread::Init() {
   // Setup the HistogramWatcher to run on the IO thread.
   net::NetworkChangeNotifier::InitHistogramWatcher();
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   globals_->extension_event_router_forwarder =
       extension_event_router_forwarder_;
 #endif