introduce DocumentParserClient to receive parser stopped notification

This depends on HTMLDocumentParser fix: https://codereview.chromium.org/532343002/

XMLHttpRequest and HTMLImport need callback when document has finished
parsing. This CL introduces an interface DocumentParserClient for receiving the
"parser stopped" notification, and migrates HTMLImportLoader to use the
interface.

BUG=409461

Review URL: https://codereview.chromium.org/535403002

git-svn-id: svn://svn.chromium.org/blink/trunk@181503 bbb929c8-8fbe-4397-9dbb-9b2b20218538