Revert "Reland: Script streaming: Add an option to make the main thread block (wait for parsing)"

This reverts commit 8b82a3d67ea45ca727c522d8e286e4cccf39ec36. This is a
speculative fix for the crashes we are seeing in Canary. This rollout is
a reland of a patch we suspect is causing the crashes. It is not clear
whether the reland fixed the crashes we were seeing.

NOTRY=true
TBR=leviw
BUG=426957

Review URL: https://codereview.chromium.org/681483002

git-svn-id: svn://svn.chromium.org/blink/trunk@184388 bbb929c8-8fbe-4397-9dbb-9b2b20218538