Deflake guarded_page_allocator_profile_test: fix a rate-limiter counter race and a test stop condition.

[copybara-service]

Deflake guarded_page_allocator_profile_test: fix a rate-limiter counter race and a test stop condition.

Two independent flakes (~0.05% each, -c opt), both in the guarded-sample rate
limiter's accounting.

1. GuardedPageAllocator::TrySample counter race (production fix).
   NeverRateLimited sets equal guarded and profile sampling intervals and asserts
   that no allocation is ever RateLimited. With equal intervals the rate-limit
   predicate reduces to num_sampled < num_guarded, which should be impossible
   because sampled allocations are a superset of guarded ones. However num_guarded
   (successful_allocations_) and num_sampled (total_sampled_count_) are incremented
   non-atomically, and by design num_sampled is incremented after num_guarded. A
   concurrent or reentrant guarded allocation can bump num_guarded before its
   num_sampled increment, so a racing TrySample reads num_sampled < num_guarded and
   spuriously returns RateLimited. Clamp the read to the documented invariant
   (num_sampled >= num_guarded). This is a no-op in steady state; at equal intervals
   it makes rate-limiting provably impossible, and it does not change behavior when
   guarded_interval > profile_interval (at the floor num_sampled == num_guarded the
   ratio is still 10, which rate-limits as intended).

2. RateLimited test stop condition (test fix).
   The test looped until the first sampled-but-not-guarded allocation and assumed
   it must be RateLimited. TrySample also returns Filtered (probabilistic
   stacktrace filter) and NoAvailableSlots, so if the first non-guarded sample was
   Filtered the window contained no RateLimited sample and the assertion failed.
   Accumulate enough guarded allocations that the 2:1 rate limiter is guaranteed to
   have produced many RateLimited samples before stopping.

Both tests still assert their original invariants (Guarded+RateLimited present
under a 2:1 interval; never RateLimited under equal intervals).