Adding ANGLE to the build includes the validation layers, which are only
for development. We don't want any development or debugging code to be
included by default in release builds, so tie building the validation
layers to enablement of asserts.

Bug: angleproject:2475
Test: gn gen out/fuchsia_rel --args='target_os="fuchsia" is_debug=false use_goma=true'
  gn desc out/fuchsia_rel //fuchsia/engine:web_engine runtime_deps

Change-Id: I6d195c680255dcbc81e95f8e6f80e1e8ace169a2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1656306Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>