Revert "try to remove google3 flag"

This reverts commit f0317e603a.

Reason for revert: breaks headless webkit, fix on the way

Original change's description:
> try to remove google3 flag
> 
> Bug: skia:
> Change-Id: I8baaf48e366932e3a90d0db57279201025c4d2b6
> Reviewed-on: https://skia-review.googlesource.com/48182
> Reviewed-by: Mike Reed <reed@google.com>

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

Change-Id: Ic331e8707531c970b577314db997b372356e6439
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/48320
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>

public.bzl

@@ -656,6 +656,7 @@ DEFINES_ALL = [
     # Required for building dm.
     "GR_TEST_UTILS",
     # Staging flags for API changes
+    "SK_SUPPORT_LEGACY_STREAM_API",
     # Should remove after we update golden images
     "SK_WEBP_ENCODER_USE_DEFAULT_METHOD",
     # Experiment to diagnose image diffs in Google3