Don't precompile scripts.

This is a reincarnation of https://chromiumcodereview.appspot.com/16034008.

Removing precompilation would be a huge win from V8 point of view, and it's not
clear how big the performance impact would be if we just remove it. The previous
time removing it was attempted, morejs regressed 5% (see crbug.com/247728), but
this regression is not reproducible any more. The benefit of precompilation in
real world use cases is questionable.

BUG=
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/177853003

git-svn-id: svn://svn.chromium.org/blink/trunk@168112 bbb929c8-8fbe-4397-9dbb-9b2b20218538