Componentize AutocompleteHistoryManager and AutofillDownload unittests.

The changes required are straightforward.

BUG=303083
R=joi@chromium.org

Review URL: https://codereview.chromium.org/187753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255039 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255039