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}

.gn

@@ -37,7 +37,7 @@ default_args = {
   # 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

DEPS

@@ -177,6 +177,12 @@ hooks = [
     'pattern': '.',
     'action': ['python', 'src/build/vs_toolchain.py', 'update'],
   },
+  {
+    # Update the Mac toolchain if necessary.
+    'name': 'mac_toolchain',
+    'pattern': '.',
+    'action': ['python', 'src/build/mac_toolchain.py'],
+  },
   # Pull binutils for linux, enabled debug fission for faster linking /
   # debugging when used with clang on Ubuntu Precise.
   # https://code.google.com/p/chromium/issues/detail?id=352046

build_overrides/build.gni

@@ -35,6 +35,5 @@ lint_suppressions_file = "//tools_webrtc/android/suppressions.xml"
 # so we just ignore that assert. See https://crbug.com/648948 for more info.
 ignore_elf32_limitations = true
 
-# Use system Xcode installation instead of the Chromium bundled Mac toolchain,
-# since it contains only SDK 10.11, not 10.12 which WebRTC needs.
-use_system_xcode = true
+# Use bundled hermetic Xcode installation maintainted by Chromium.
+use_system_xcode = false