[omnibox] Cleans up zps related features

omnibox::kReactiveZeroSuggestionsOnNTPOmnibox
omnibox::kReactiveZeroSuggestionsOnNTPRealbox

remote zps is enabled by default on all platforms (with conditions).
Thus, these two features are no-ops in that context and can be safely
removed. local history zps is enabled by default on Desktop and
Android and disabled by default on iOS. These two features allow for
local history zps to be enabled when rZPS is enabled which in
retrospect is unnecessary given that these two types of zps are
orthogonal. Thus they can be safely removed in that context as well.

omnibox::kOmniboxLocalZeroSuggestForAuthenticatedUsers
omnibox::kOmniboxZeroSuggestCaching

These two default-enabled features were used to experiment with
disabling local history zps backfill for signed-in users and remote
zps caching. A series of changes resolved the issues with the local
backfill.We are also no longer pursuing disabling caching for remote
zps due to its negative effect on the metrics. Instead we will be
experimenting with a prefetching mechanism. Thus these two features
can also be safely removed.

Fixed: 1071877
Change-Id: I16c291267ddee7fd5762265974d7fe21fa814d7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2840087
Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#875227}
GitOrigin-RevId: 5084e3163515f8bfaeb6c57528a5ecde89f05b20
1 file changed