Revert of Postpone deletion of (Password)AutofillAgent (patchset #1 id:1 of https://codereview.chromium.org/1943873002/ )

Reason for revert:
This fix is not complete, I need to also handle disabling the LegacyAutofillAgent.

Because the final fix is going to be merged, I prefer to have the complete solution in one CL. Therefore I am reverting this partial fix, and will reintroduce it in a new CL. Sorry for the noise.

BUG=609010, 609007, 608100, 608101

Original issue's description:
> Postpone deletion of (Password)AutofillAgent
>
> Currently, the two agent classes delete themselves immediately on destruction
> of the RenderFrame they are observing. This CL postpones the deletion to a
> separately posted task, to avoid the situation when the agent is deleted while
> still having methods in progress lower on the stack.
>
> BUG=608100
>
> Committed: https://crrev.com/a8755e432460c9412291c0ae4dd887babb3fa506
> Cr-Commit-Position: refs/heads/master@{#391236}

TBR=mathp@chromium.org,dvadym@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=608100

Review-Url: https://codereview.chromium.org/1951813002
Cr-Commit-Position: refs/heads/master@{#391465}
4 files changed