Fix assert in c++/BUILD.gn

BUG=801780
R=thakis

Change-Id: I60588de2dd5de1fe92f9731592d7ebd4468764c2
Reviewed-on: https://chromium-review.googlesource.com/c/1479410
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633807}

build/config/c++/BUILD.gn

@@ -68,7 +68,8 @@ config("runtime_library") {
     # implements the Itanium C++ ABI, and not the Microsoft ABI which we use on
     # Windows (and we need to use in order to interoperate correctly with COM
     # among other things).
-    assert(!is_win, "Don't use libcxxabi on Windows.")
+    assert(!export_libcxxabi_from_executables,
+           "Don't use libcxxabi on Windows.")
 
     cflags_cc +=
         [ "-I" + rebase_path("$libcxx_prefix/include", root_build_dir) ]