Check if the RenderView is alive before setting the plugin find handler

Presumably during destruction, the RenderView can be destroyed before
the RenderFrame (since it only holds a weak pointer to it). We need
to check whether it is alive before clearing the plugin find handler.

BUG=487146

Review URL: https://codereview.chromium.org/1123393004

Cr-Commit-Position: refs/heads/master@{#330320}

content/renderer/pepper/pepper_plugin_instance_impl.cc

@@ -663,7 +663,7 @@ v8::Local<v8::Context> PepperPluginInstanceImpl::GetMainWorldContext() {
 void PepperPluginInstanceImpl::Delete() {
   is_deleted_ = true;
 
-  if (render_frame_ &&
+  if (render_frame_ && render_frame_->render_view() &&
       render_frame_->render_view()->plugin_find_handler() == this) {
     render_frame_->render_view()->set_plugin_find_handler(NULL);
   }