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.

libcef_dll/cpptoc/response_cpptoc.cc

@@ -9,7 +9,7 @@
 // implementations. See the translator.README.txt file in the tools directory
 // for more information.
 //
-// $hash=4573a140180f11230e688f73e8c09503f9123c3d$
+// $hash=5c6b14610ef7bcadf5e4936a262c660896a32526$
 //
 
 #include "libcef_dll/cpptoc/response_cpptoc.h"
@@ -119,10 +119,7 @@ void CEF_CALLBACK response_set_status_text(struct _cef_response_t* self,
   DCHECK(self);
   if (!self)
     return;
-  // Verify param: statusText; type: string_byref_const
-  DCHECK(statusText);
-  if (!statusText)
-    return;
+  // Unverified params: statusText
 
   // Execute
   CefResponseCppToC::Get(self)->SetStatusText(CefString(statusText));
@@ -150,10 +147,7 @@ void CEF_CALLBACK response_set_mime_type(struct _cef_response_t* self,
   DCHECK(self);
   if (!self)
     return;
-  // Verify param: mimeType; type: string_byref_const
-  DCHECK(mimeType);
-  if (!mimeType)
-    return;
+  // Unverified params: mimeType
 
   // Execute
   CefResponseCppToC::Get(self)->SetMimeType(CefString(mimeType));
@@ -181,10 +175,7 @@ void CEF_CALLBACK response_set_charset(struct _cef_response_t* self,
   DCHECK(self);
   if (!self)
     return;
-  // Verify param: charset; type: string_byref_const
-  DCHECK(charset);
-  if (!charset)
-    return;
+  // Unverified params: charset
 
   // Execute
   CefResponseCppToC::Get(self)->SetCharset(CefString(charset));
@@ -275,10 +266,7 @@ void CEF_CALLBACK response_set_url(struct _cef_response_t* self,
   DCHECK(self);
   if (!self)
     return;
-  // Verify param: url; type: string_byref_const
-  DCHECK(url);
-  if (!url)
-    return;
+  // Unverified params: url
 
   // Execute
   CefResponseCppToC::Get(self)->SetURL(CefString(url));