GN: Fix iOS error in audio_device and rtc_base

With this in, the only compilation errors left seems
related to yasm and libjpeg_turbo.
Notice the below example builds x86 builds (not ARM) since if
specifying target_cpu="arm", the gn step fails (separate issue).

BUG=webrtc:5213, webrtc:5195, chromium:459705
TESTED=Passing compilation with:
gn gen --args="target_os=\"ios\"" out/Default
ninja -C out/Default rtc_base audio_device

Review URL: https://codereview.webrtc.org/1471663002

Cr-Commit-Position: refs/heads/master@{#10763}

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
+# 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",

webrtc/modules/audio_device/BUILD.gn

@@ -50,6 +50,7 @@ source_set("audio_device") {
     include_dirs += [ "android" ]
   }
   defines = []
+  cflags = []
   if (rtc_include_internal_audio_device) {
     defines += [ "WEBRTC_INCLUDE_INTERNAL_AUDIO_DEVICE" ]
     sources += [