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This is gated by omnibox::kOmniboxTrendingZeroPrefixSuggestionsOnNTP

With the change the platform-specific defaults for signed-out users will
change as follows:

- Desktop: no request -> remote suggestions.
- Android: Most Visited -> remote suggestions.
- iOS: N/A. Continues to request Most Visited.

Bug: 1122669
Change-Id: I5b226c7d5d56912a93f3d6ac4b1e295e7c50e07b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388157Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803916}