Clear Map::constructor to object_function from the same context for prototype maps. This avoids keeping small pockets of memory alive for the common pattern where prototypes are used to emulate classes:

function inherit(parent, child) {
  function p() {}
  p.prototype = parent.prototype;
  child.prototype = new p();
}

Otherwise child.prototype[constructor] keeps alive p, p's context, the
initial map attached to p, and the (now empty) transition array of the
initial map.

BUG=

Review URL: https://codereview.chromium.org/942493002

Cr-Commit-Position: refs/heads/master@{#26747}