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_response.h

@@ -96,7 +96,7 @@ class CefResponse : public virtual CefBaseRefCounted {
   ///
   // Set the response status text.
   ///
-  /*--cef()--*/
+  /*--cef(optional_param=statusText)--*/
   virtual void SetStatusText(const CefString& statusText) = 0;
 
   ///
@@ -108,7 +108,7 @@ class CefResponse : public virtual CefBaseRefCounted {
   ///
   // Set the response mime type.
   ///
-  /*--cef()--*/
+  /*--cef(optional_param=mimeType)--*/
   virtual void SetMimeType(const CefString& mimeType) = 0;
 
   ///
@@ -120,7 +120,7 @@ class CefResponse : public virtual CefBaseRefCounted {
   ///
   // Set the response charset.
   ///
-  /*--cef()--*/
+  /*--cef(optional_param=charset)--*/
   virtual void SetCharset(const CefString& charset) = 0;
 
   ///
@@ -150,7 +150,7 @@ class CefResponse : public virtual CefBaseRefCounted {
   ///
   // Set the resolved URL after redirects or changed as a result of HSTS.
   ///
-  /*--cef()--*/
+  /*--cef(optional_param=url)--*/
   virtual void SetURL(const CefString& url) = 0;
 };