Don't post multiple task for executing blocked scripts.
We may post a lot of tasks for executing blocked scripts in the case
where we insert multiple shadow trees each containing a style element.
When we start parsing a style element, we mark it as blocking and
unblock script execution after finishing parsing. Check if the previous
task is active before posting.
Found while measuring performance for issue 603621.
BUG=603621
Review-Url: https://codereview.chromium.org/2609763002
Cr-Commit-Position: refs/heads/master@{#441110}
1 file changed
