linux: fix race in TransportDIB passing

chrome/plugin/webplugin_proxy.cc

Index: chrome/plugin/webplugin_proxy.cc
diff --git a/chrome/plugin/webplugin_proxy.cc b/chrome/plugin/webplugin_proxy.cc
index 0e2e006f47760656b78ad1734fcc6e62b08ca8d0..8a046502687fb86360455103d28ca932da30d0b5 100644
--- a/chrome/plugin/webplugin_proxy.cc
+++ b/chrome/plugin/webplugin_proxy.cc
@@ -636,7 +636,13 @@ void WebPluginProxy::SetWindowlessBuffer(
   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));