Do not handle DoSetCursor for deleted plugins.

Fix a crash related to change
https://chromium-review.googlesource.com/c/chromium/src/+/2418998

Seems there is reentrancy during destruction for pepper plugin
instance. Do not access any containers if we've been marked for
deletion. This follows the same convention for handling messages.

BUG=1130991

Change-Id: I2004265dc5592761c24c957be6231c3a1ad86d2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2423271Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809429}

content/renderer/pepper/pepper_plugin_instance_impl.cc

@@ -3028,6 +3028,9 @@ void PepperPluginInstanceImpl::SetAlwaysOnTop(bool on_top) {
 }
 
 void PepperPluginInstanceImpl::DoSetCursor(std::unique_ptr<ui::Cursor> cursor) {
+  if (is_deleted_)
+    return;
+
   cursor_ = std::move(cursor);
   if (fullscreen_container_) {
     fullscreen_container_->PepperDidChangeCursor(*cursor_);