[rAc] [OSX] Fix padding beneath Autofill dialog title.

The padding was too large when there were no notifications visible.  Fixed by
tweaking how different sections define their paddings.

Also, removed a TODO about moving notifications into the AutofillHeader class.
After trying this, I decided that they're actually better where they are now,
due to the fact that the AutofillHeader class is also used in conjunction with
the sign-in view, for which no notifications are visible (but we still want to
reserve vertical space for them when sizing the sign-in view so that the
dialog's size doesn't bounce around).

BUG=315977
TEST=Open rAc dialog, switch to local Autofill mode, enter data, confirm.
     Then open dialog again (should default to local Autofill mode with no
     notifications visible).  The padding below the dialog title should be
     20px rather than 30px.
R=groby@chromium.org

Review URL: https://codereview.chromium.org/111353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240067 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240067