Revert "remove guard for computeByteSize api"

This reverts commit d72747c5c3b3aaaa9e5957242bd478980cac942e.

Reason for revert: broke //htmlrender/webkit_headless/engine/testing:platform_web_unittests

Original change's description:
> remove guard for computeByteSize api
> 
> Bug: skia:
> Change-Id: I5c66a7c79c4e71ec82194e8d37de04a516028665
> Reviewed-on: https://skia-review.googlesource.com/56402
> Reviewed-by: Mike Reed <reed@google.com>

TBR=reed@google.com,reed@chromium.org

Change-Id: If721607583204df9f32e54a041ecc8b34ad1a749
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/56321
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/public.bzl b/public.bzl
index e832550..9b3afbe 100644
--- a/public.bzl
+++ b/public.bzl
@@ -656,6 +656,7 @@
     # Required for building dm.
     "GR_TEST_UTILS",
     # Staging flags for API changes
+    "SK_SUPPORT_LEGACY_SAFESIZE64",
     # Should remove after we update golden images
     "SK_WEBP_ENCODER_USE_DEFAULT_METHOD",
     # Experiment to diagnose image diffs in Google3