Add debug information on failing CanHandleType

TBR=fukino@chromium.org
notry=true

Bug: 804413
Change-Id: I6fd7c66036d38bf23c6b77e16f214eff06c7d903
Reviewed-on: https://chromium-review.googlesource.com/980939Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546069}

chrome/browser/chromeos/extensions/file_manager/private_api_file_system.cc

@@ -807,7 +807,9 @@ bool FileManagerPrivateInternalResolveIsolatedEntriesFunction::RunAsync() {
   for (size_t i = 0; i < params->urls.size(); ++i) {
     const FileSystemURL file_system_url =
         file_system_context->CrackURL(GURL(params->urls[i]));
-    DCHECK(external_backend->CanHandleType(file_system_url.type()));
+    DCHECK(external_backend->CanHandleType(file_system_url.type()))
+        << "GURL: " << file_system_url.ToGURL()
+        << "type: " << file_system_url.type();
     FileDefinition file_definition;
     const bool result =
         file_manager::util::ConvertAbsoluteFilePathToRelativeFileSystemPath(