Emit an error log line when attempting to connect to an app at an invalid URL.

mojo/shell/dynamic_application_loader.cc

Index: mojo/shell/dynamic_application_loader.cc
diff --git a/mojo/shell/dynamic_application_loader.cc b/mojo/shell/dynamic_application_loader.cc
index e1baea487dd0ed42dd5a15cc3fab5e2291bca6e9..ed66e1f42499d1fc35512db7800fc4442bd4ad7d 100644
--- a/mojo/shell/dynamic_application_loader.cc
+++ b/mojo/shell/dynamic_application_loader.cc
@@ -211,6 +211,8 @@ DynamicApplicationLoader::~DynamicApplicationLoader() {
 void DynamicApplicationLoader::RegisterContentHandler(
     const std::string& mime_type,
     const GURL& content_handler_url) {
+  DCHECK(content_handler_url.is_valid())
+    << "Content handler URL is invalid for mime type " << mime_type;
   mime_type_to_url_[mime_type] = content_handler_url;
 }