Fix a browser crash when an extension with multiple active pages crashes.

chrome/browser/extensions/extensions_service.cc

 // Copyright (c) 2010 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/extensions/extensions_service.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/command_line.h"
@@ skipping 1374 matching lines @@
       NewRunnableMethod(
           backend_.get(), &ExtensionsServiceBackend::CheckForExternalUpdates,
           killed_extensions, scoped_refptr<ExtensionsService>(this)));
 }
 
 void ExtensionsService::UnloadExtension(const std::string& extension_id) {
   // Make sure the extension gets deleted after we return from this function.
   scoped_refptr<const Extension> extension(
       GetExtensionByIdInternal(extension_id, true, true));
 
-  // Callers should not send us nonexistent extensions.
-  CHECK(extension.get());
+  // This method can be called via PostTask, so the extension may have been
+  // unloaded by the time this runs.
+  if (!extension)
+    return;
 
   // Keep information about the extension so that we can reload it later
   // even if it's not permanently installed.
   unloaded_extension_paths_[extension->id()] = extension->path();
 
   // Clean up if the extension is meant to be enabled after a reload.
   disabled_extension_paths_.erase(extension->id());
 
   // Clean up runtime data.
   extension_runtime_data_.erase(extension_id);
@@ skipping 535 matching lines @@
 }
 
 bool ExtensionsService::IsBeingUpgraded(const Extension* extension) {
   return extension_runtime_data_[extension->id()].being_upgraded;
 }
 
 void ExtensionsService::SetBeingUpgraded(const Extension* extension,
                                          bool value) {
   extension_runtime_data_[extension->id()].being_upgraded = value;
 }