Reland 2: Script streaming: Add an option to make the main thread block (wait for parsing)

This is the same as r184361 which was speculatively reverted (since the earlier
version of it was broken).

This makes the main thread block and wait for the parser thread when all the
script data has arrived. The goal is to ensure that the script (which can now be
compiled) will get the main thread's attention as soon as possible.

This feature is temporary: The usefulness of blocking will be evaluated with
Finch, and this option will be removed (blocking will be always enabled or never
enabled, based on which option turns out to be better).

Previous (broken) version: https://codereview.chromium.org/651163002/

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/674133002

git-svn-id: svn://svn.chromium.org/blink/trunk@184504 bbb929c8-8fbe-4397-9dbb-9b2b20218538