Have AutofillManagerDelegate supply the AutofillWebDataService to core code.

In this change, Autofill core code now gets the AutofillWebDataService that it
should use from its embedder (i.e., AutofillManagerDelegate) rather than by
calling AutofillWebDataService::FromBrowserContext().

This change is a prelude to eliminating the
AutofillWebDataService::FromBrowserContext() method in favor of static methods
that live in the embedders as necessary.

BUG=303048

Review URL: https://codereview.chromium.org/71683003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235994 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235994