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

This reverts r184277.

Reason: ASSERTS fail in debug mode because ScriptStreamer gets destroyed while
holding the mutex.

BUG=
TBR=marja@chromium.org, haraken@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/680493002

git-svn-id: svn://svn.chromium.org/blink/trunk@184349 bbb929c8-8fbe-4397-9dbb-9b2b20218538