Ignore Return and Cmd+Return in the omnibox if no text has been entered.

chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.h"
 
 #include <Carbon/Carbon.h>  // kVK_Return
 
 #include "base/auto_reset.h"
 #include "base/mac/foundation_util.h"
@@ skipping 799 matching lines @@
     return model()->AcceptKeyword(KeywordModeEntryMethod::TAB);
   }
 
   // |-noop:| is sent when the user presses Cmd+Return. Override the no-op
   // behavior with the proper WindowOpenDisposition.
   NSEvent* event = [NSApp currentEvent];
   if (cmd == @selector(insertNewline:) ||
      (cmd == @selector(noop:) &&
       ([event type] == NSKeyDown || [event type] == NSKeyUp) &&
       [event keyCode] == kVK_Return)) {
+    // If the user hasn't entered any text in keyword search mode, we need to
+    // return early in order to avoid cancelling the search.
+    if (GetTextLength() == 0)
+      return true;
+
     WindowOpenDisposition disposition =
         ui::WindowOpenDispositionFromNSEvent(event);
     model()->AcceptInput(disposition, false);
     // Opening a URL in a background tab should also revert the omnibox contents
     // to their original state.  We cannot do a blanket revert in OpenURL()
     // because middle-clicks also open in a new background tab, but those should
     // not revert the omnibox text.
     RevertAll();
     return true;
   }
@@ skipping 252 matching lines @@
                               display_text);
   NSDictionary* notification_info = @{
       NSAccessibilityAnnouncementKey : announcement,
       NSAccessibilityPriorityKey :     @(NSAccessibilityPriorityHigh)
   };
   NSAccessibilityPostNotificationWithUserInfo(
       [field_ window],
       NSAccessibilityAnnouncementRequestedNotification,
       notification_info);
 }