Use bundled SDK 10.12 toolchain for Mac and iOS.

BUG=webrtc:6608, webrtc:8099, chromium:659726, chromium:763440
TBR=mbonadei@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2995033002
Cr-Commit-Position: refs/heads/master@{#19753}
diff --git a/.gn b/.gn
index 0ee19e9..2fe0610 100644
--- a/.gn
+++ b/.gn
@@ -37,7 +37,7 @@
   # component builds.
   is_component_build = false
 
-  mac_sdk_min = "10.11"
+  mac_sdk_min = "10.12"
 
   # WebRTC does not want to switch to C++14 yet.
   use_cxx11 = true