Reintroduce the ICF flag, turn it on for bots

This was brought up on https://gn-review.googlesource.com/c/gn/+/3741,
some of GN users use linkers that don't support ICF so we need to
provide an option to disable it rather than enabling it unconditionally.

Change-Id: I8300f2b1883468a586ca26f358c6fdfc1c93afb5
Reviewed-on: https://gn-review.googlesource.com/c/3820
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Petr Hosek <phosek@google.com>
diff --git a/infra/recipes/gn.expected/ci_win.json b/infra/recipes/gn.expected/ci_win.json
index a565841..764f2fa 100644
--- a/infra/recipes/gn.expected/ci_win.json
+++ b/infra/recipes/gn.expected/ci_win.json
@@ -246,7 +246,8 @@
       "python",
       "-u",
       "[START_DIR]\\gn\\build\\gen.py",
-      "--use-lto"
+      "--use-lto",
+      "--use-icf"
     ],
     "cwd": "[START_DIR]\\gn",
     "env": {