Set empty icon to alert dialogs on mac

chrome/browser/ui/cocoa/javascript_app_modal_dialog_cocoa.mm

Index: chrome/browser/ui/cocoa/javascript_app_modal_dialog_cocoa.mm
diff --git a/chrome/browser/ui/cocoa/javascript_app_modal_dialog_cocoa.mm b/chrome/browser/ui/cocoa/javascript_app_modal_dialog_cocoa.mm
index fa6d65f95eb8f30e4324317d593a1f0a81d7f58b..51f96a0da3fc25cf35ab3df7b303203e179243ed 100644
--- a/chrome/browser/ui/cocoa/javascript_app_modal_dialog_cocoa.mm
+++ b/chrome/browser/ui/cocoa/javascript_app_modal_dialog_cocoa.mm
@@ -81,8 +81,13 @@ const int kMessageTextMaxSlots = 2000;
 }
 
 - (NSAlert*)alert {
-  if (!alert_)
+  if (!alert_) {
     alert_.reset([[NSAlert alloc] init]);
+    // Set a blank icon.
+    NSImage* image =
+        [[[NSImage alloc] initWithSize:NSMakeSize(1, 1)] autorelease];
+    [alert_ setIcon:image];
+  }
   return alert_;
 }