Fix crash when using Instant.

chrome/browser/ui/cocoa/tab_contents/previewable_contents_controller.mm

 // Copyright (c) 2011 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.
 
 #import "chrome/browser/ui/cocoa/tab_contents/previewable_contents_controller.h"
 
 #include "base/logging.h"
 #include "base/mac/bundle_locations.h"
 #include "base/mac/mac_util.h"
 #include "content/public/browser/web_contents.h"
@@ skipping 31 matching lines @@
   // There may be no previewContents_ in the prerender case.
   if (!previewContents_)
     return;
 
   // Remove the preview contents.
   [previewContents_->GetNativeView() removeFromSuperview];
   previewContents_->WasHidden();
   previewContents_ = nil;
 }
 
+- (void)onInsertTabWithContents:(WebContents*)contents {
+  if (previewContents_ == contents) {
+    [previewContents_->GetNativeView() removeFromSuperview];
+    previewContents_ = nil;
+  }
+}
+
 - (BOOL)isShowingPreview {
   return previewContents_ != nil;
 }
 
 @end