add SK_SUPPORT_LEGACY_BITMAP_GETTEXTURE for out-of-date google3 callers

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2177993002

Review-Url: https://codereview.chromium.org/2177993002
diff --git a/public.bzl b/public.bzl
index eb0909f..569308d 100644
--- a/public.bzl
+++ b/public.bzl
@@ -590,6 +590,7 @@
     "GOOGLE3",
     # Staging flags for API changes
     "SK_SUPPORT_LEGACY_ACCESSBITMAP",
+    "SK_SUPPORT_LEGACY_BITMAP_GETTEXTURE",
     "SK_SUPPORT_LEGACY_COLORFILTER_PTR",
     "SK_SUPPORT_LEGACY_CREATESHADER_PTR",
     "SK_SUPPORT_LEGACY_IMAGEFACTORY",