Always use the limited entropy provider for studies constrained to the limited layer.

The logic in VariationsLayers::SelectEntropyProviderForStudy is
short-circuiting the entropy selection causing the default entropy
provider to be used when a study without Google web experiment ID is
constrained to a limited layer.

This change removes the short-circuiting by checking whether the study must use the limited entropy provider first. The positioning of the logic will override any checks for default and low entropy. OTOH only the check for session entropy can override limit entropy.

Bug: 41491646
Change-Id: I5fe91e443a6d97f39b4f2aeb52451e57edcabfdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5335039
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Yulun Zeng <yulunz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1271696}