Fix issues with request callbacks during browser shutdown (see issue #2622).

The behavior has changed as follows with NetworkService enabled:
- All pending and in-progress requests will now be aborted when the CEF context
  or associated browser is destroyed. The OnResourceLoadComplete callback will
  now also be called in this case for in-progress requests that have a handler.
- The CefResourceHandler::Cancel method will now always be called when resource
  handling is complete, irrespective of whether handling completed successfully.
- Request callbacks that arrive after the OnBeforeClose callback for the
  associated browser (which may happen for in-progress requests that are aborted
  on browser destruction) will now always have a non-nullptr CefBrowser
  parameter.
- Allow empty parameters to CefRequest and CefResponse methods where it makes
  sense (e.g. resetting default response state, or clearing a referrer value).
- Fixed a reference loop that was keeping CefResourceHandler objects from being
  destroyed if they were holding a callback reference (from ProcessRequest,
  ReadResponse, etc.) during CEF context or associated browser destruction.
- Fixed an issue where the main frame was not detached on browser destruction
  which could cause a crash due to RFH use-after-free (see issue #2498).

To test: All unit tests pass as expected.

include/cef_life_span_handler.h

@@ -195,9 +195,13 @@ class CefLifeSpanHandler : public virtual CefBaseRefCounted {
   ///
   // Called just before a browser is destroyed. Release all references to the
   // browser object and do not attempt to execute any methods on the browser
-  // object after this callback returns. This callback will be the last
-  // notification that references |browser|. See DoClose() documentation for
-  // additional usage information.
+  // object (other than GetIdentifier or IsSame) after this callback returns.
+  // This callback will be the last notification that references |browser| on
+  // the UI thread. Any in-progress network requests associated with |browser|
+  // will be aborted when the browser is destroyed, and
+  // CefResourceRequestHandler callbacks related to those requests may still
+  // arrive on the IO thread after this method is called. See DoClose()
+  // documentation for additional usage information.
   ///
   /*--cef()--*/
   virtual void OnBeforeClose(CefRefPtr<CefBrowser> browser) {}