GN: Fix iOS error in audio_device and rtc_base

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index 540058b01178f50d9f147756a52932a7aaa36a55..0a7c17ee8adb41655e6198929643cc3da5833ff2 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -10,6 +10,13 @@ import("//build/config/crypto.gni")
 import("//build/config/ui.gni")
 import("../build/webrtc.gni")
 
+# Enable OpenSSL (BoringSSL) for iOS. This is covered in webrtc/supplement.gypi

[kjellander_webrtc, 2015/11/23 09:18:36]

This is globally applied for GYP with supplement.gypi but here it's kept much
closer to where it's used, which is by design in GN (and is great).

And if I try to put this into any of the common configs of
https://chromium.googlesource.com/external/webrtc/+/master/webrtc/BUILD.gn I get
this when running gn gen:
ERROR at //webrtc/BUILD.gn:95:21: Assignment had no effect.
      use_openssl = true
                    ^---
You set the variable "use_openssl" here and it was unused before it went
out of scope.

+# for the GYP build.
+import("//build_overrides/webrtc.gni")
+if (is_ios && !build_with_chromium) {
+  use_openssl = true
+}
+
 config("rtc_base_config") {
   include_dirs = [
     "//third_party/jsoncpp/overrides/include",