Moved the threaded parser data receiver functionality behind a flag

This is done to temporarily unbreak the Inspector network instrumentation until a better fix can be landed.

R=abarth
BUG=398076

Review URL: https://codereview.chromium.org/414123009

git-svn-id: svn://svn.chromium.org/blink/trunk@179292 bbb929c8-8fbe-4397-9dbb-9b2b20218538