Have SearchProvider look up the omnibox start margin directly.

chrome/browser/ui/omnibox/omnibox_controller.cc

Index: chrome/browser/ui/omnibox/omnibox_controller.cc
diff --git a/chrome/browser/ui/omnibox/omnibox_controller.cc b/chrome/browser/ui/omnibox/omnibox_controller.cc
index 31ae5e59ecfbe6f7a40cefae391a6c1ce5ead5b8..a15dd1435760f43724ed1ba328b7089b39c3b00d 100644
--- a/chrome/browser/ui/omnibox/omnibox_controller.cc
+++ b/chrome/browser/ui/omnibox/omnibox_controller.cc
@@ -43,16 +43,10 @@ void OmniboxController::StartAutocomplete(
     AutocompleteInput::PageClassification current_page_classification,
     bool prevent_inline_autocomplete,
     bool prefer_keyword,
-    bool allow_exact_keyword_match,
-    int omnibox_start_margin) const {
+    bool allow_exact_keyword_match) const {
   ClearPopupKeywordMode();
   popup_->SetHoveredLine(OmniboxPopupModel::kNoMatch);
 
-  if (chrome::IsInstantExtendedAPIEnabled()) {
-    autocomplete_controller_->search_provider()->
-        SetOmniboxStartMargin(omnibox_start_margin);
-  }
-
   // We don't explicitly clear OmniboxPopupModel::manually_selected_match, as
   // Start ends up invoking OmniboxPopupModel::OnResultChanged which clears it.
   autocomplete_controller_->Start(AutocompleteInput(