[iOS/GN] Allow compilation with system clang.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 9f564794816655edd06d110aeb2ce14390f037bb..c7d2ceed5aa4671d99b4c1b18ad86ac2797a66fe 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -26,6 +26,11 @@ declare_args() {
     # Clang compiler version. Clang files are placed at version-dependent paths.
     clang_version = "3.9.0"
   }
+
+  # Compile with Xcode version of clang instead of hermetic version shipped
+  # with the build. Used on iOS to ship official builds (as they are built
+  # with the version of clang shipped with Xcode).
+  use_xcode_clang = is_ios && is_official_build
 }
 
 # Subdirectory within root_out_dir for shared library files.