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_request.h

@@ -99,7 +99,7 @@ class CefRequest : public virtual CefBaseRefCounted {
   // fully qualified with an HTTP or HTTPS scheme component. Any username,
   // password or ref component will be removed.
   ///
-  /*--cef()--*/
+  /*--cef(optional_param=referrer_url)--*/
   virtual void SetReferrer(const CefString& referrer_url,
                            ReferrerPolicy policy) = 0;
 
@@ -154,7 +154,7 @@ class CefRequest : public virtual CefBaseRefCounted {
   // existing values will not be overwritten. The Referer value cannot be set
   // using this method.
   ///
-  /*--cef()--*/
+  /*--cef(optional_param=value)--*/
   virtual void SetHeaderByName(const CefString& name,
                                const CefString& value,
                                bool overwrite) = 0;
@@ -193,7 +193,7 @@ class CefRequest : public virtual CefBaseRefCounted {
   // Set the URL to the first party for cookies used in combination with
   // CefURLRequest.
   ///
-  /*--cef()--*/
+  /*--cef(optional_param=url)--*/
   virtual void SetFirstPartyForCookies(const CefString& url) = 0;
 
   ///