[IOS] Clean up iframe messaging support base::Feature code

Bug: 887985,881364
Change-Id: I40d6b2f6e57c1a3059d266e7d6093712d1d2b9ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525042
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643043}
25 files changed