[omnibox] Removed tooltip related override

OmniboxPopupContentsView squelched any queries for tooltips by returning
null from GetTooltipHandlerForPoint(). This change removes it. Doesn't
seem to make a difference, but tooltips now work for the tab switch
button and result view more generally.

Bug: 853929
Change-Id: I995af95362bca165495052444b1432ee875fc5e7
Reviewed-on: https://chromium-review.googlesource.com/1155744
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Kevin Bailey <krb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579452}