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 7c578714da68d306bd905e0c66197143c0dc9ea3..0b0e8fdd9ba98ff51501617ecdfc6f69d801a266 100644
--- a/mojo/shell/dynamic_application_loader.cc
+++ b/mojo/shell/dynamic_application_loader.cc
@@ -362,6 +362,8 @@ 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;
 }