[Autofill] Remove forms with same rendererID from FormCache.

While extracting forms and adding them to the FormCache, modified forms
are considered as new forms. However, we don't detect if a form is
removed from the DOM.

This CL prevents readding forms with the same rendererID by removing the
old form with the same rendererID.

Bug: 1215333
Change-Id: I7f0cead34c76d36111d437b0555470f17b7b1fb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3015847
Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Eva Herencsárová <evih@google.com>
Cr-Commit-Position: refs/heads/master@{#900925}