Make USE_EXTERNAL_POPUP_MENU local to content.

content/renderer/BUILD.gn

Index: content/renderer/BUILD.gn
diff --git a/content/renderer/BUILD.gn b/content/renderer/BUILD.gn
index f2a9aa5dc708c745c2209bb5c7634456cddaa2d5..eca7dc87b47cb2fb5b4ad5aab16ed41a7454db1b 100644
--- a/content/renderer/BUILD.gn
+++ b/content/renderer/BUILD.gn
@@ -6,6 +6,7 @@ import("//build/config/chromecast_build.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
+import("//content/common/features.gni")
 import("//media/media_options.gni")
 import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
@@ -428,6 +429,7 @@ target(link_target_type, "renderer") {
     "//content:resources",
     "//content/child",
     "//content/common",
+    "//content/common:features",
     "//content/gpu:gpu_sources",
     "//content/public/child:child_sources",
     "//content/public/common:common_sources",