Enable BackgroundHTMLParser for HTML imports

Before this CL, Parsing HTML imports were pinned to main thread.
This CL enables BackgroundHTMLParser for parsing HTML imports.

Layout test fast/css/resources/import-layout-with-pending-sheet.html
is modified to set dumpAsText() earlier to address flakes.

TEST=fast/html/imports
BUG=421289

Review URL: https://codereview.chromium.org/636013002

git-svn-id: svn://svn.chromium.org/blink/trunk@183387 bbb929c8-8fbe-4397-9dbb-9b2b20218538