Turn on /OPT:REF and /OPT:ICF too.

These two together shave another 5MB off dm.exe, from 16MB -> 11MB.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3738

Change-Id: Id216867e0ad5bc115fbd4006095860dff9204947
Reviewed-on: https://skia-review.googlesource.com/3738
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Ben Wagner <bungeman@google.com>

gn/BUILD.gn

@@ -333,6 +333,10 @@ config("release") {
       "/O2",
       "/Zc:inline",
     ]
+    ldflags = [
+      "/OPT:ICF",
+      "/OPT:REF",
+    ]
   } else {
     cflags = [
       "-O3",