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

Review-Url: https://codereview.chromium.org/1943873002
Cr-Commit-Position: refs/heads/master@{#391236}