exernalize imagefactory guard

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1897453002

TBR=
CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot,Build-Mac-Clang-x86_64-Release-CMake-Trybot

Review URL: https://codereview.chromium.org/1897453002
diff --git a/public.bzl b/public.bzl
index ad5d231..aea55c6 100644
--- a/public.bzl
+++ b/public.bzl
@@ -554,6 +554,7 @@
     "SK_SUPPORT_LEGACY_PATHEFFECT_PTR",
     "SK_SUPPORT_LEGACY_PICTURE_PTR",
     "SK_SUPPORT_LEGACY_MASKFILTER_PTR",
+    "SK_SUPPORT_LEGACY_IMAGEFACTORY",
     "SK_SUPPORT_LEGACY_XFERMODE_PTR",
 ]