Add iOS static library GN build script.

webrtc/build/webrtc.gni

Index: webrtc/build/webrtc.gni
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index ac03959fc581cb2330a5e2f725b9ab5f07c42a19..ced30459e25f3c2dc537bc06caf038ae155ba170 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -132,6 +132,10 @@ declare_args() {
   # Build sources requiring GTK. NOTICE: This is not present in Chrome OS
   # build environments, even if available for Chromium builds.
   rtc_use_gtk = !build_with_chromium
+
+  # Enable this to make all symbols visible when building static library
+  # targets.
+  rtc_override_visibility = false
 }
 
 # A second declare_args block, so that declarations within it can
@@ -212,6 +216,10 @@ set_defaults("rtc_static_library") {
 set_defaults("rtc_shared_library") {
   configs = rtc_add_configs
   suppressed_configs = []
+  if (rtc_override_visibility) {
+    configs += [ "//build/config/gcc:symbol_visibility_default" ]
+    suppressed_configs += [ "//build/config/gcc:symbol_visibility_hidden" ]
+  }
 }
 
 template("rtc_test") {