Routing ID from ChromePluginPlaceholder::CreateRoutingId() is used without a process context.

Example #1: Sending an IPC to a potentially wrong process in PluginObserver::PluginPlaceholderHost::DownloadFinished

Example #2: Using the routing ID as a key for PluginObserver::plugin_placeholders_ map (which AFAIU can be populated based on input from multiple processes).

Example #3: Key in PluginObserver::component_observers_ map

Example #4: Sending an IPC to a potentially wrong process in PluginObserver::ComponentObserver::OnEvent


Notes:

- The list of problems above is not necessary exhaustive / complete.

- This bug is a follow-up for a CR comment at https://chromium-review.googlesource.com/c/chromium/src/+/630416/4/chrome/browser/plugins/plugin_observer.cc#140