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Currently, ZeroSuggestProvider uses DEFAULT_SERP_FOR_URL by default.

After this CL, ZeroSuggestProvider will use NONE as the default.

And there will be a ZeroSuggestVariant value of "RemoteSendURL" that
can be queried via:
OmniboxFieldTrial::InZeroSuggestRemoteSendURLFieldTrial()

The objective of this is twofold:

 1) Enable us to switch put certain page classifications into the
    DEFAULT_SERP_FOR_URL mode without putting every page classification
    into this. This is needed for On Focus Query
    Refinement.

 2) Disables ZeroSuggestProvider by default for all page
    classifications not in this field trial. This will allow us to
    get rid of the fake ZeroSuggestRedirectToChrome URL method of
    disabling ZeroSuggestProvider, which is not obvious.

    This is also fairly important for On Focus Query Refinement, as
    then we will want to specify a real endpoint URL, not a fake one
    to disable the feature.

Bug: 963173
Change-Id: I61ce4bbf2f228b477bdf82c581b4cedd88fa546b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632920Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665959}