CefRequestContext leaks

[cztomczak]

Upstream:
A. https://bitbucket.org/chromiumembedded/cef/issues/2505/cefrequestcontext-inherited-from-global
B. https://magpcss.org/ceforum/viewtopic.php?f=6&t=16267

In cefpython code:

1. In CreateBrowserSync:

   cefpython/src/cefpython.pyx

   Line 750 in 6f5bf08

   # Request context - part 1/2.

2. In CefShutdown:

   cefpython/src/cefpython.pyx

   Line 946 in 6f5bf08

   # Release shared request context. In the past this was sometimes