Refactor process execution logic (see issue #2969)

CefContext implements the public CEF API functions and delegates
the stages of content & service_manager process execution to
CefMainRunner. CEF-specific runtime logic (which may be replaced
with chrome-specific runtime logic) is then delegated to
CefMainDelegate which implements content::ContentMainDelegate.

libcef/browser/chrome_browser_process_stub.cc

@@ -62,7 +62,7 @@ void ChromeBrowserProcessStub::OnContextInitialized() {
   context_initialized_ = true;
 }
 
-void ChromeBrowserProcessStub::Shutdown() {
+void ChromeBrowserProcessStub::CleanupOnUIThread() {
   CEF_REQUIRE_UIT();
   DCHECK(initialized_);
   DCHECK(context_initialized_);