[turbofan] Improve typing of ToBoolean.

According to ES6 draft, revision 29 (2014-12-06), section 7.1.2,
ToBoolean yields true for true, detectable receivers (the ES6 Object
type), symbols and numbers except -0, +0 and NaN.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/807663006

Cr-Commit-Position: refs/heads/master@{#25915}