Use ScopedFieldTrials in FieldTrialsTest

Resetting the global state between runs was previously handled by a
RAII type, but the semantics of that type changed to remove this
behavior in [1].

[1] https://webrtc-review.googlesource.com/c/src/+/276269

Bug: webrtc:14731, webrtc:14705
Change-Id: I8425cb71f49ea000434d500e0b3978324e4c3195
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/285782
Commit-Queue: Emil Lundmark <lndmrk@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38800}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index dfb2aa1..7d76433 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -1367,6 +1367,7 @@
       "../rtc_base/containers:flat_set",
       "../rtc_base/task_utils:repeating_task",
       "../system_wrappers:field_trial",
+      "../test:field_trial",
       "../test:fileutils",
       "../test:rtc_expect_death",
       "../test:test_support",