Fix crash from ipc_fuzzer

Can't reproduce original crash so I just fix all crashes I see.

Use WeakPtr in AskUserForSettings callback.

Move chrome::ShowMessageBox in separate task and protection against nested message boxes.

Add missing return into AskUserForSettings.

BUG=472146

Review URL: https://codereview.chromium.org/1083433003

Cr-Commit-Position: refs/heads/master@{#324746}
4 files changed