limit visibility of opts and optional targets

These targets are all implementation details of :skia.

Cq-Include-Trybots: skia.primary:Build-Debian9-Clang-arm-Release-Flutter_Android
Change-Id: Ife5c0923bf37363516908e524dbea92be67cb965
Reviewed-on: https://skia-review.googlesource.com/c/169223
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index d62b0bd..d5dd3ee 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -217,6 +217,7 @@
 
 # Use for CPU-specific Skia code that needs particular compiler flags.
 template("opts") {
+  visibility = [ ":*" ]
   if (invoker.enabled) {
     source_set(target_name) {
       forward_variables_from(invoker, "*")
@@ -331,6 +332,7 @@
 
 # Any feature of Skia that requires third-party code should be optional and use this template.
 template("optional") {
+  visibility = [ ":*" ]
   if (invoker.enabled) {
     config(target_name + "_public") {
       if (defined(invoker.public_defines)) {