Enable whole-program virtual function optimization in LTO mode.

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index a6b3add86d6cc360e785b451ff12c94134f4386f..9a8d1fe26975d0c18fd20445580686f117bbe861 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -9,7 +9,7 @@ import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
 # This value will be inherited in the toolchain below.
-if (is_lto) {
+if (allow_posix_link_time_opt || is_cfi) {
   concurrent_links =
       exec_script("get_concurrent_links.py", [ "--lto" ], "value")
 } else {
@@ -217,7 +217,8 @@ template("gcc_toolchain") {
     tool("alink") {
       rspfile = "{{output}}.rsp"
       arflags = ""
-      if (is_lto && invoker.toolchain_os != "nacl") {
+      if ((allow_posix_link_time_opt || is_cfi) &&
+          invoker.toolchain_os != "nacl") {
         gold_plugin_path = rebase_path(
                 "//third_party/llvm-build/Release+Asserts/lib/LLVMgold.so",
                 root_build_dir)