[omnibox] Move "ZeroSuggestVariant" to be a context sensitive param

Currently for ZeroSuggestProvider, "ZeroSuggestVariant" is simply
globally configured.

This CL moves "ZeroSuggestParam" to operate using the existing
GetValueForRuleInContext, which allows it to be parameterized by
the page classification.

In other words, this CL allows us to configure different
On Focus Suggestion modes per page classification.

This will likely be used to put the search results page in a custom
query refinement On Focus Suggestion mode.

Bug: 963173
Change-Id: If7df3c4a7fc4906788f0a466b7a6edeccd614b8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617208
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661999}
6 files changed