Revert "hide xfermode for g3"

This reverts commit a5b2376365681f3c341bc4ff74bf0caedbfe9359.

Reason for revert: breaking google3

Original change's description:
> hide xfermode for g3
> 
> BUG=skia:
> 
> GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5132
> 
> Change-Id: I374abc04856e27df718145f9ae57cc83a447ba3e
> Reviewed-on: https://skia-review.googlesource.com/5132
> Reviewed-by: Mike Reed <reed@google.com>
> Commit-Queue: Mike Reed <reed@google.com>
> 

TBR=reed@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I115bc7814d6c96d897b8ae86a917ce2365c6b0b4
Reviewed-on: https://skia-review.googlesource.com/5185
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/public.bzl b/public.bzl
index 2513955..55f40f8 100644
--- a/public.bzl
+++ b/public.bzl
@@ -599,6 +599,7 @@
     # Staging flags for API changes
     "SK_SUPPORT_LEGACY_ACCESSBITMAP",
     "SK_SUPPORT_LEGACY_CLIP_REGIONOPS",
+    "SK_SUPPORT_LEGACY_XFERMODE_IS_PUBLIC",
     # Temporarily Disable analytic AA for Google3
     "SK_NO_ANALYTIC_AA",
 ]