Have SearchProvider look up the omnibox start margin directly.

chrome/browser/autocomplete/search_provider.cc

Index: chrome/browser/autocomplete/search_provider.cc
diff --git a/chrome/browser/autocomplete/search_provider.cc b/chrome/browser/autocomplete/search_provider.cc
index d4245fd34e8719b14b02a9213e22a51447052195..d474eefe26269e7baf422b3aae7034bf2d955e18 100644
--- a/chrome/browser/autocomplete/search_provider.cc
+++ b/chrome/browser/autocomplete/search_provider.cc
@@ -36,6 +36,10 @@
 #include "chrome/browser/search_engines/template_url_prepopulate_data.h"
 #include "chrome/browser/search_engines/template_url_service.h"
 #include "chrome/browser/search_engines/template_url_service_factory.h"
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/browser_finder.h"
+#include "chrome/browser/ui/browser_instant_controller.h"
+#include "chrome/browser/ui/search/instant_controller.h"
 #include "chrome/common/net/url_fixer_upper.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/common/url_constants.h"
@@ -250,8 +254,7 @@ SearchProvider::SearchProvider(AutocompleteProviderListener* listener,
       providers_(TemplateURLServiceFactory::GetForProfile(profile)),
       suggest_results_pending_(0),
       field_trial_triggered_(false),
-      field_trial_triggered_in_session_(false),
-      omnibox_start_margin_(-1) {
+      field_trial_triggered_in_session_(false) {
 }
 
 // static
@@ -369,10 +372,6 @@ void SearchProvider::ResetSession() {
   field_trial_triggered_in_session_ = false;
 }
 
-void SearchProvider::SetOmniboxStartMargin(int omnibox_start_margin) {
-  omnibox_start_margin_ = omnibox_start_margin;
-}
-
 SearchProvider::~SearchProvider() {
 }
 
@@ -1382,11 +1381,25 @@ void SearchProvider::AddMatchToMap(const string16& query_string,
                                    int accepted_suggestion,
                                    bool is_keyword,
                                    MatchMap* map) {
+  int omnibox_start_margin = chrome::kDisableStartMargin;
+// browser_finder does not exist for Android. On IOS, start margin is not used.

[Peter Kasting, 2013/08/09 21:03:46]

Nit: To avoid unusual comment indenting, how about:

  // On non-mobile, ask the instant controller for the appropriate start margin.
  // On Android, FindBrowserWithProfile() doesn't exist, and on iOS, the start
  // margin is unused, so leave the value as default there.
  int omnibox_start_margin = chrome::kDisableStartMargin;
#if ...

You can simplify this comment if we don't use the start margin on Android
anyway, by just saying that this is unused on mobile.

[jeremycho, 2013/08/09 23:55:09]

Done.

+#if !defined(OS_ANDROID) && !defined(IOS)
+  if (chrome::IsInstantExtendedAPIEnabled()) {
+    Browser* browser =
+        chrome::FindBrowserWithProfile(profile_, chrome::GetActiveDesktop());
+    if (browser && browser->instant_controller() &&
+        browser->instant_controller()->instant()) {
+      omnibox_start_margin =
+          browser->instant_controller()->instant()->omnibox_bounds().x();
+    }
+  }
+#endif  // !defined(OS_ANDROID) && !defined(IOS)
+
   const TemplateURL* template_url = is_keyword ?
       providers_.GetKeywordProviderURL() : providers_.GetDefaultProviderURL();
   AutocompleteMatch match = CreateSearchSuggestion(this, relevance, type,
       template_url, query_string, input_text, input_, is_keyword,
-      accepted_suggestion, omnibox_start_margin_,
+      accepted_suggestion, omnibox_start_margin,
       !is_keyword || providers_.default_provider().empty());
   if (!match.destination_url.is_valid())
     return;