Enable whole-program virtual function optimization in LTO mode.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index e7f43792af1b4c76ce0f7666b583ed267a60f69e..41daacc667b0ce96a0425b5f6a46256e549f509e 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -5,6 +5,14 @@
 # Toolchain-related configuration that may be needed outside the context of the
 # toolchain() rules themselves.
 
+declare_args() {
+  # Enable Link Time Optimization in optimized builds (output programs run
+  # faster, but linking is up to 5-20x slower).
+  #
+  # TODO(pcc): Remove this flag if/when LTO is enabled in official builds.
+  allow_posix_link_time_opt = false
+}
+
 # Subdirectory within root_out_dir for shared library files.
 # TODO(agrieve): GYP sets this to "lib" for Linux & Android, but this won't work
 #     in GN until support for loadable_module() is added.