Revert "[ios] Remove call to NSUserDefaults synchronize"

This reverts commit d3778f410c36db17987a83dcfe2bfa5e11539e4b.

Reason for revert: It's safer to keep that code for now

Original change's description:
> [ios] Remove call to NSUserDefaults synchronize
>
> Apple documentation discourages use of synchronize because unnecessary.
> https://developer.apple.com/documentation/foundation/nsuserdefaults/1414005-synchronize
>
> Bug: none
> Change-Id: I769e11b43cf40f6588d1e0263fe759e41d684aa2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5975529
> Auto-Submit: Federica Germinario <fedegermi@google.com>
> Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1377637}

Bug: none
Change-Id: I62ba5db9390c336cb61b135f73a686a9db9dff5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6018308
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Auto-Submit: Federica Germinario <fedegermi@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1382985}