Make libyuv use the hermetic xcode toolchain.

This is what Chromium is moving to, and what WebRTC is using.
Next step: pull xcode via CIPD.

Bug:chromium:633032
Change-Id: I6a21aa0e5b212a3a9cb4e7cc79ebb614a89f73ed
Reviewed-on: https://chromium-review.googlesource.com/915921
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@chromium.org>

.gn

@@ -29,7 +29,7 @@ exec_script_whitelist = build_dotfile_settings.exec_script_whitelist +
                         [ "//build_overrides/build.gni" ]
 
 default_args = {
-  mac_sdk_min = "10.11"
+  mac_sdk_min = "10.12"
 
   # LibYUV does not want to switch to C++14 yet.
   use_cxx11 = true

DEPS

@@ -197,6 +197,12 @@ 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

@@ -33,6 +33,14 @@ ubsan_vptr_blacklist_path =
 # 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 maintained by Chromium,
+# except for local iOS builds where it is unsupported.
+if (host_os == "mac") {
+  _result = exec_script("//build/mac/should_use_hermetic_xcode.py",
+                        [ target_os ],
+                        "value")
+  assert(_result != 2,
+         "Do not allow building targets with the default" +
+             "hermetic toolchain if the minimum OS version is not met.")
+  use_system_xcode = _result == 0
+}