Revert 169577 "[HTML Import] Let script block by pending resourc..."

This is a speculative revert to address reported crashes in the wild.

> [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

TBR=morrita@chromium.org

Review URL: https://codereview.chromium.org/201673005

git-svn-id: svn://svn.chromium.org/blink/trunk@169769 bbb929c8-8fbe-4397-9dbb-9b2b20218538