History: Fix and re-enable the singleDeletion browser test.

The problem was that the test was waiting for any element to be deleted
from the DOM, but deleting a single history entry would result in several
DOM elements being deleted. The fix is to add a callback that is only
called when everything has been deleted.

BUG=242293
R=bauerb@chromium.org

Review URL: https://codereview.chromium.org/64123003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234477 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234477