History: Delete day header when all entries on a day have been removed.

When deleting individual history entries (using dropdown menu on desktop,
or the "X" button on mobile), the header for a day should be deleted at
as soon as the last entry for the day is deleted.

BUG=260823
TEST=See repro steps in bug.
R=sergiu@chromium.org

Review URL: https://codereview.chromium.org/68883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234522 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234522