iwa: Use `IsIwaDevModeEnabled` in `IsolatedWebAppTrustChecker`

Bug: 40254511, 272716897
Change-Id: I47763b2ab3555e2eff655588a73128b10393fa4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5331576


Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Commit-Queue: Christian Flach <cmfcmf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267203}