Output android_secondary_abi_toolchain to build_vars.txt

Bug: 877510
Change-Id: Icf50fe4d6102a0b87ead889e161cafff3c1eb348
Reviewed-on: https://chromium-review.googlesource.com/1188869
Commit-Queue: Ben Mason <benmason@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586004}

build/android/BUILD.gn

@@ -54,6 +54,11 @@ if (enable_java_templates) {
       "android_ndk_root=" + rebase_path(android_ndk_root, root_build_dir) + CR
   _data += "android_tool_prefix=" +
            rebase_path(android_tool_prefix, root_build_dir) + CR
+  if (defined(android_secondary_abi_cpu)) {
+    _secondary_label_info = get_label_info(":foo($android_secondary_abi_toolchain)", "root_out_dir")
+    _data += "android_secondary_abi_toolchain=" +
+             rebase_path(_secondary_label_info, root_build_dir) + CR
+  }
   write_file(android_build_vars, _data)
 }