[iOS] Fix compilation of official build with Xcode 8.

build/config/mac/mac_sdk.gni

Index: build/config/mac/mac_sdk.gni
diff --git a/build/config/mac/mac_sdk.gni b/build/config/mac/mac_sdk.gni
index eb0fdda61492d095fc13f6586316e4b11162610f..587f43ddbe337f7f18a487b2f7d0bcab516ab00e 100644
--- a/build/config/mac/mac_sdk.gni
+++ b/build/config/mac/mac_sdk.gni
@@ -25,8 +25,20 @@ declare_args() {
   mac_sdk_name = "macosx"
 }
 
+# Check that the version of macOS SDK used is the one requested when building
+# a version of Chrome shipped to the users. Disable the check if building for
+# iOS as the version macOS SDK used is not relevant for the tool build for the
+# host (they are not shipped) --- this is required as Chrome on iOS is usually
+# build with the latest version of Xcode that may not ship with the version of
+# the macOS SDK used to build Chrome on mac.
+# TODO(crbug.com/635745): the check for target_os should be replaced by a
+# check that current_toolchain is default_toolchain, and the file should
+# assert that current_os is "mac" once this file is no longer included by
+# iOS toolchains.
+_verify_sdk = is_chrome_branded && is_official_build && target_os != "ios"
+
 find_sdk_args = [ "--print_sdk_path" ]
-if (is_chrome_branded && is_official_build) {
+if (_verify_sdk) {
   find_sdk_args += [
     "--verify",
     mac_sdk_min,