Revert of Revert of [Autofill] Merge the autofill_dates table into the autofill table. (https://codereview.chromium.org/184183002/)

Reason for revert:
Reverting DB changes breaks users who've updated their DB since then, so the original revert was probably a mistake.  Will fix the test soon in a follow-up CL.

Original issue's description:
> Revert of [Autofill] Merge the autofill_dates table into the autofill table. (https://codereview.chromium.org/166313002/)
> 
> Reason for revert:
> Broke the GeneralMixMemoryTest.* target from the performance_ui_tests suite.
> 
> BUG=347340
> 
> Original issue's description:
> > [Autofill] Merge the autofill_dates table into the autofill table.
> > 
> > We already only store two timestamps per autocomplete entry: creation and last
> > use.  However, we used to store an arbitrary number of timestamps, and the table
> > structure was designed to support that.  Now that that's no longer the case, we
> > can dramatically simplify the code.
> > 
> > The one cost to only keeping two timestamps, rather than all of them, is that
> > it's no longer possible to perfectly undo changes in response to a user clearing
> > Autofill data for e.g. the last week.  In practice, users are more likely to
> > care about having less data stored on disk and traversing the Sync pipes than
> > about having perfect frequency information after partially clearing the Autofill
> > data.
> > 
> > BUG=118696
> > TEST=unit_tests
> > R=estade@chromium.org
> > 
> > Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=253380
> 
> TBR=estade@chromium.org,pkasting@chromium.org,phajdan.jr@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=118696
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=254059

TBR=estade@chromium.org,pkasting@chromium.org,phajdan.jr@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=347340

Review URL: https://codereview.chromium.org/184953003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254256 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254256