add legacy flag for google3

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2234373002

TBR=

Review-Url: https://codereview.chromium.org/2234373002
diff --git a/public.bzl b/public.bzl
index c409c39..4c3e70b 100644
--- a/public.bzl
+++ b/public.bzl
@@ -617,6 +617,7 @@
     "SK_SUPPORT_LEGACY_PICTURE_PTR",
     "SK_SUPPORT_LEGACY_TYPEFACE_PTR",
     "SK_SUPPORT_LEGACY_XFERMODE_PTR",
+    "SK_SUPPORT_LEGACY_PICTUREINSTALLPIXELREF",
 ]
 
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