[Autofill] Fixed circular dependency in driver destruction.

This CL fixes a circular dependency: ~ContentAutofillDriverFactory()
destroys ContentAutofillDriverFactory::router_ and afterwards calls
~AutofillDriverFactory(), which implicitly deletes all drivers and thus
calls ~ContentAutofillDriver(), which again refers to
ContentAutofillDriverFactory::router_.

This CL also eliminates ContentAutofillRouter::Reset() to simplify
the destruction API.

Bug: 1187842
Change-Id: I2c904292a771b757db806a24de732768251e4fb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3137732


Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#918733}