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Currently, AutocompleteResult doesn't really support the no-default-match
case. The first match is set to be the default match, even if none of the
matches are |allowed_to_be_default_match|.

This is problematic, because there's three known cases where there's
genuinely no default match:
 - NTP ZeroSuggest
 - Enterprise policy where there's no default search provider
 - ChromeOS launcher

ChromeOS launcher already supports the no-default-match case properly.
The other two are subtly broken.

This CL does two things:
 - It updates AutocompleteResult to set default_match() == end() when
   there's no valid default match.
 - It updates a bunch of logic OmniboxPopupModel (runs on Views and iOS)
   to support the no-default-match case.

Bug: 1016845, 363656
Change-Id: Ib684efba92b05cb677bdabf9265e35e2bfbeab56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906882
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714664}