[omnibox, MacViews] Open background tab on command-enter.

Previously, this opened a new tab in the foreground. Opening in a
background tab matches the behavior of Cocoa Chrome and Safari.

Bug: 838966
Change-Id: I5220c614afab0fc1894af0f39caa350529ef7634
Reviewed-on: https://chromium-review.googlesource.com/1049949Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556794}

chrome/browser/ui/views/omnibox/omnibox_view_views.cc

@@ -1141,10 +1141,15 @@ bool OmniboxViewViews::HandleKeyEvent(views::Textfield* textfield,
               OmniboxPopupModel::TAB_SWITCH) {
         popup_view_->OpenMatch(WindowOpenDisposition::SWITCH_TO_TAB);
       } else {
-        model()->AcceptInput(alt || command
-                                 ? WindowOpenDisposition::NEW_FOREGROUND_TAB
-                                 : WindowOpenDisposition::CURRENT_TAB,
-                             false);
+        if (alt) {
+          model()->AcceptInput(WindowOpenDisposition::NEW_FOREGROUND_TAB,
+                               false);
+        } else if (command) {
+          model()->AcceptInput(WindowOpenDisposition::NEW_BACKGROUND_TAB,
+                               false);
+        } else {
+          model()->AcceptInput(WindowOpenDisposition::CURRENT_TAB, false);
+        }
       }
       return true;