SkBitmap::setPixelRef():  less reference churn

BUG=skia:

Change-Id: I9657e4af5fcc808f9175ff336155374ccc77999d
Reviewed-on: https://skia-review.googlesource.com/5461
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/public.bzl b/public.bzl
index 666a598..147f11b 100644
--- a/public.bzl
+++ b/public.bzl
@@ -594,6 +594,7 @@
     # Temporarily Disable analytic AA for Google3
     "SK_NO_ANALYTIC_AA",
     "SK_SUPPORT_LEGACY_CLIPOPS_PLAIN_ENUM",
+    "SK_SUPPORT_LEGACY_BITMAP_SETPIXELREF",
 ]
 
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