Make USE_EXTERNAL_POPUP_MENU local to content.

content/common/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("features.gni")
+import("//build/buildflag_header.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//ppapi/features/features.gni")
 import("//sandbox/features.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
 if (is_mac) {
   import("//build/config/mac/mac_sdk.gni")
 }
 
+# For feature flags internal to content. See content/public/common:features
+# for feature flags that clients of contents need to know about.
+buildflag_header("features") {
+  header = "features.h"
+
+  flags = [ "USE_EXTERNAL_POPUP_MENU=$use_external_popup_menu" ]
+}
+
 source_set("common") {
   # Targets external to content should always link to the public API.
   # In addition, targets outside of the content component (shell and tests)
   # must not link to this because it will duplicate the code in the component
   # build.
   visibility = [ "//content/*" ]
 
   sources = [
     "accessibility_messages.h",
     "all_messages.h",
@@ skipping 323 matching lines @@
   ]
 
   public_deps = [
     ":mojo_bindings",
     "//cc",
     "//gpu/command_buffer/common",
     "//ipc",
     "//third_party/WebKit/public:blink_headers",
   ]
   deps = [
+    ":features",
     "//base",
     "//base/third_party/dynamic_annotations",
     "//build/util:webkit_version",
     "//cc/ipc",
     "//cc/surfaces",
     "//components/discardable_memory/common",
     "//components/tracing",
     "//components/tracing:startup_tracing",
     "//content:resources",
     "//content/app/resources",
@@ skipping 259 matching lines @@
     "//url/mojo:url_mojom_origin",
   ]
 
   overridden_deps = [ "//ipc:mojom" ]
   component_deps = [ "//ipc" ]
 
   export_class_attribute = "CONTENT_EXPORT"
   export_define = "CONTENT_IMPLEMENTATION=1"
   export_header = "content/common/content_export.h"
 }