Dynamically insrted HTMLImport shouldn't block script

This change let <link rel=import> not block the script unless
it is inserted by the parser. The change is straightforward:
Add a flag to indicate whether the import is added by a parser,
and check if before turning the blocking flag on.

BUG=328793
TEST=import-async.html,import-async-tree.html,import-async-shared.html
R=dglazkov,dominicc

Review URL: https://codereview.chromium.org/113703002

git-svn-id: svn://svn.chromium.org/blink/trunk@164323 bbb929c8-8fbe-4397-9dbb-9b2b20218538