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_resource_request_handler.h

@@ -174,6 +174,13 @@ class CefResourceRequestHandler : public virtual CefBaseRefCounted {
   // |response| represent the request and response respectively and cannot be
   // modified in this callback. |status| indicates the load completion status.
   // |received_content_length| is the number of response bytes actually read.
+  // This method will be called for all requests, including requests that are
+  // aborted due to CEF shutdown or destruction of the associated browser. In
+  // cases where the associated browser is destroyed this callback may arrive
+  // after the CefLifeSpanHandler::OnBeforeClose callback for that browser. The
+  // CefFrame::IsValid method can be used to test for this situation, and care
+  // should be taken not to call |browser| or |frame| methods that modify state
+  // (like LoadURL, SendProcessMessage, etc.) if the frame is invalid.
   ///
   /*--cef(optional_param=browser,optional_param=frame)--*/
   virtual void OnResourceLoadComplete(CefRefPtr<CefBrowser> browser,