The Fuchsia platform build and the Chromium one use a completely
different set of configs to specify default warnings. The previous
version of BUILD.gn worked with Chromium, but failed to build
with Fuchsia.

This CL fixes the issue by identifying said configs and reusing
them consistently.

Change-Id: I7de526a57d2f14eb93c03e06401d2c3059d35e9a