Add support for providers called when the omnibox is focused.
The feature described in https://docs.google.com/document/d/1vXHSui-1QYjemBHOpiLiAsIWEIoKuiw66xNcNVfUC3A/edit
requires an omnibox provider that is called when the omnibox is focused.
Right now, only the zero_suggest_provider_ can be called on focus. This CL
makes any provider returning true for |ActivatedOnOmniboxFocus| be called on
focus.
BUG=228111
TBR=miguelg
Review URL: https://codereview.chromium.org/
820063002
Cr-Commit-Position: refs/heads/master@{#310085}