[Autofill]: Add AutofillProfileEnabled preference.

This creates and uses AutofillProfileEnabled flag to disable profile autofill.
This affects:
- PersonalDatamanager
- FormDataImporter
- AutofillManager

AutofillProfileEnabled will be added as a policy as a follow up CL.

Bug: 860526
Change-Id: Ied9a29c9ada7b7e564f1ba19fa738854fab7b17e
Reviewed-on: https://chromium-review.googlesource.com/1127344
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574186}
18 files changed