[tvOS] Fix UKM recorder feedback loop in single-process web tests

tvOS runs web tests single-process, so registering the mojo UKM recorder
as a DelegatingUkmRecorder delegate makes its IPC loop back to the same
singleton, amplifying UKM entries exponentially and starving JS timers.

Blink cannot read switches::kSingleProcess directly, so
WebTestContentRendererClient reads it and pushes it down via a new
SetIsRunningSingleProcessWebTest setter, mirroring the existing
SetWebTestMode pattern. WebTestSupport checks that flag before
registering the delegate, so this also covers other platforms run with
single-process for debugging, not just tvOS.

Bug: 545671292
Change-Id: I7579c9a4784b012983d8882ed8c01eb16dfe96b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8257449
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1682804}