Add callbacks for unresponsive render process (fixes #3661)

The client can optionally wait or terminate the render process.

Expose process exit codes via OnRenderProcessTerminated and
CefGetExitCode (fixes #2126).

cefclient: Add a new https://tests/hang page for testing hang behavior.

cefclient: Move message and resource handling to a new BaseClientHandler
class to support loading of test pages in default Chrome UI windows.

tests/ceftests/message_router_unittest_utils.cc

@@ -184,7 +184,9 @@ void MRTestHandler::OnBeforeClose(CefRefPtr<CefBrowser> browser) {
 }
 
 void MRTestHandler::OnRenderProcessTerminated(CefRefPtr<CefBrowser> browser,
-                                              TerminationStatus status) {
+                                              TerminationStatus status,
+                                              int error_code,
+                                              const CefString& error_string) {
   message_router_->OnRenderProcessTerminated(browser);
 }