[build] Enable LTO by default in non-debug builds

This patch turns on LTO by default in non-debug builds
as a followup patch to https://fxrev.dev/1001661.

Bug: 42182779
Run-All-Tests: true
Cq-Include-Trybots: luci.turquoise.global.try:run-postsubmit-tryjobs
Change-Id: Idca2b141d6c1feb2f3bf8d3a0c66225e9ed41090
diff --git a/build/config/compiler.gni b/build/config/compiler.gni
index 6ac5c84..ddab20de 100644
--- a/build/config/compiler.gni
+++ b/build/config/compiler.gni
@@ -6,7 +6,7 @@
   # TODO(https://fxbug.dev/42170458): consider "debug"
   default_optimize = "none"
 } else {
-  default_optimize = "size"
+  default_optimize = "size_lto"
 }
 
 # NOTE: Keep in sync with //build/config/zircon/levels.gni