linux: fix race in TransportDIB passing

chrome/plugin/webplugin_proxy.cc

 // Copyright (c) 2009 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/plugin/webplugin_proxy.h"
 
 #include "build/build_config.h"
 #if defined(OS_LINUX)
 #include <gtk/gtk.h>
 #endif
@@ skipping 618 matching lines @@
 #elif defined (OS_LINUX)
 void WebPluginProxy::UpdateTransform() {
 }
 
 void WebPluginProxy::SetWindowlessBuffer(
     const TransportDIB::Handle& windowless_buffer,
     const TransportDIB::Handle& background_buffer) {
   int width = delegate_->GetRect().width();
   int height = delegate_->GetRect().height();
   windowless_dib_.reset(TransportDIB::Map(windowless_buffer));
-  windowless_canvas_.reset(windowless_dib_->GetPlatformCanvas(width, height));
+  if (windowless_dib_.get()) {
+    windowless_canvas_.reset(windowless_dib_->GetPlatformCanvas(width, height));
+  } else {
+    // This can happen if the renderer has already destroyed the TransportDIB
+    // by the time we receive the handle, e.g. in case of multiple resizes.
+    windowless_canvas_.reset();
+  }
   background_dib_.reset(TransportDIB::Map(background_buffer));
   if (background_dib_.get()) {
     background_canvas_.reset(background_dib_->GetPlatformCanvas(width, height));
   } else {
     background_canvas_.reset();
   }
 }
 #endif
 
 void WebPluginProxy::CancelDocumentLoad() {
@@ skipping 32 matching lines @@
   while (index != resource_clients_.end()) {
     WebPluginResourceClient* client = (*index).second;
 
     if (client == resource_client) {
       resource_clients_.erase(index++);
     } else {
       index++;
     }
   }
 }