[HTML Import] Let script block by pending resources created by lifecycle callback

This change tells the HTML parser know that lifecycle callbacks,
that is triggered through Microtask checkpoint, can create resources that
blocks scripts.

It also streamlines the unblock notification from Document to the parser
into didLoadAllScriptBlockingResources(). Now both didRemoveAllPendingStylesheets()
and didLoadAllImports() checks resource readiness and invokes the unified notification.

TEST=custom-element-style-block.html
BUG=346408
R=dglazkov@chromium.org, dominicc@chromium.org

Review URL: https://codereview.chromium.org/202813002

git-svn-id: svn://svn.chromium.org/blink/trunk@169577 bbb929c8-8fbe-4397-9dbb-9b2b20218538