Add a variable use_system_xcode to GN.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 6251fe05947f988ceff0163112de1da0f7c751a6..dab5b77c7c9f8d6ef820acdadbce9ad71f6d222b 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -40,6 +40,18 @@ declare_args() {
   # 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
+
+  # Use the system install of Xcode for tools like ibtool, libtool, etc.
+  # This does not affect the compiler. When this variable is false, targets will
+  # instead use a hermetic install of Xcode. [The hermetic install can be
+  # obtained with gclient sync after setting the environment variable
+  # FORCE_MAC_TOOLCHAIN].
+  use_system_xcode = true
+
+  # The path to the hermetic install of Xcode. Only relevant when
+  # use_system_xcode = false.
+  hermetic_xcode_path =

[sdefresne, 2016/10/07 13:25:16]

nit: do you expect developer to change this variable? if not, maybe it would be
better to put it outside of "declare_args()" block

[erikchen, 2016/10/07 23:25:41]

I expect to use this when I rev the hermetic toolchain. Not sure about others...

+      rebase_path("//build/mac_files/Xcode.app", "", root_build_dir)
 }
 
 declare_args() {