[ios] Feature flag for XHR submission detection

Context: go/bling-xhr-submission-detection

Bug: 328271347
Change-Id: Ib1ce521c04eecd1c80f98d2dbb9c1d969b341e3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5352708


Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Ernesto Izquierdo Clua <eic@google.com>
Cr-Commit-Position: refs/heads/main@{#1270956}