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These are fixes that are needed to enable Autofill in iframes.
It also add more tests for the frame messaging logic.

Bug: 881364
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Iea417c1d371094489daca66e7477d963d003c716
Reviewed-on: https://chromium-review.googlesource.com/1234333
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593282}