Convert enable_plugins to a buildflag header.

content/browser/loader/mime_sniffing_resource_handler.cc

Index: content/browser/loader/mime_sniffing_resource_handler.cc
diff --git a/content/browser/loader/mime_sniffing_resource_handler.cc b/content/browser/loader/mime_sniffing_resource_handler.cc
index 3e5e414ff1c08445630977b3f31a4f795a4513ab..fb70d6ef5924a8ba8dff5466487105ea0574db4d 100644
--- a/content/browser/loader/mime_sniffing_resource_handler.cc
+++ b/content/browser/loader/mime_sniffing_resource_handler.cc
@@ -36,6 +36,7 @@
 #include "net/http/http_content_disposition.h"
 #include "net/http/http_response_headers.h"
 #include "net/url_request/url_request.h"
+#include "ppapi/features/features.h"
 #include "url/origin.h"
 
 namespace content {
@@ -74,7 +75,7 @@ MimeSniffingResourceHandler::MimeSniffingResourceHandler(
     : LayeredResourceHandler(request, std::move(next_handler)),
       state_(STATE_STARTING),
       host_(host),
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
       plugin_service_(plugin_service),
 #endif
       must_download_(false),
@@ -425,7 +426,7 @@ bool MimeSniffingResourceHandler::CheckForPluginHandler(
     bool* defer,
     bool* handled_by_plugin) {
   *handled_by_plugin = false;
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
   ResourceRequestInfoImpl* info = GetRequestInfo();
   bool allow_wildcard = false;
   bool stale;