[PSA] Work commencing to remove BrowserPlugin

[W. James Maclean]

Now that PDFs have transitioned to using out-of-process iframes (OOPIF), we are going to begin the process of removing BrowserPlugin and its related codepaths (see https://cs.chromium.org/chromium/src/content/renderer/browser_plugin/browser_plugin.h?rcl=756bf743b4d0d8fdcd9d8eeb2722a55b0075e43c&l=40).

This should not affect consumers of <webview> or the various GuestView derivatives, as these have all transitioned to using the  OOPIF architecture, but if you have C++ code that directly relies on BrowserPlugin you will need to convert it to using OOPIF.

Please feel free to contact me with questions.

Regards,

James