Cronet: set clang_use_default_sample_profile via GN arg declaration

At the time clang_use_default_sample_profile was introduced, there was
no GN arg to define whether Cronet was being built. Now that this
exists it's preferable to rely on that instead of override.

Additionally, now that we have dropped all GN arg overrides for Cronet
bots (expect for is_cronet_build), make it clear that no new overrides
should ever be added.

Bug: 40287068
Change-Id: Icddc5a5edd1f2743d33e3db0d1012cffdf9fdd74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6964624
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Stefano Duo <stefanoduo@google.com>
Reviewed-by: Etienne Dechamps <edechamps@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1517940}