[Autofill iOS] Add guards when accessing iframe.

These methods can be calle asynchronously.
It is necessary to check that the web_frame was not destroyed before
processing it.

Bug: 942427
Change-Id: Icc46dae9193c4fd9ef9612ce47239bd50eef33d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538194
Commit-Queue: Olivier Robin <[email protected]>
Reviewed-by: Javier Ernesto Flores Robles <[email protected]>
Reviewed-by: Moe Ahmadi <[email protected]>
Cr-Commit-Position: refs/heads/master@{#644779}
1 file changed