Add NetworkService support for CefURLRequest (see issue #2622).

Requests created using CefURLRequest::Create are not associated with a
browser/frame. When originating from the render process these requests cannot be
intercepted and consequently only http(s) and blob requests are supported. To
work around this limitation a new CefFrame::CreateURLRequest method has been
added that allows the request to be associated with that browser/frame for
interception purposes.

This change also fixes an issue with the NetworkService implementation where
redirected requests could result in two parallel requests being sent to the
target server.

To test: URLRequestTest.* tests pass with NetworkService enabled.

libcef/browser/browser_host_impl.h

@@ -351,9 +351,6 @@ class CefBrowserHostImpl : public CefBrowserHost,
                 int script_start_line,
                 CefRefPtr<CefResponseManager::Handler> responseHandler);
 
-  void ExecuteJavaScriptWithUserGestureForTests(int64 frame_id,
-                                                const CefString& javascript);
-
   // Open the specified text in the default text editor.
   void ViewText(const std::string& text);