Cleanup: Remove most enable_extensions logic in extensions code.

extensions/BUILD.gn

Index: extensions/BUILD.gn
diff --git a/extensions/BUILD.gn b/extensions/BUILD.gn
index 1c9602ea57963f06beee3595b61397e911ff527f..b44ac8928ba80222f562a29cd47a0ef1534be2f5 100644
--- a/extensions/BUILD.gn
+++ b/extensions/BUILD.gn
@@ -5,6 +5,8 @@
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
+assert(enable_extensions)
+
 # GYP version: extensions/extensions_resources.gyp:extensions_resources
 group("extensions_resources") {
   deps = [
@@ -48,15 +50,11 @@ grit("extensions_renderer_resources") {
   ]
   grit_flags = [ "-E", "mojom_root=" + rebase_path(root_gen_dir) ]
 
-  # TODO(GYP,cjhopman): GYP has this dependency guarded by enable_extensions==1,
-  # but I don't know how that possibly works.
   deps = [
     "//device/serial:serial_mojo",
   ]
 }
 
-if (enable_extensions) {
-
 source_set("test_support") {
   testonly = true
   sources = [
@@ -277,5 +275,3 @@ test("extensions_unittests") {
   }
 }
 }
-
-}