Clear Map::constructor to object_function from the same context for prototype maps. This avoids kee…

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=

Committed: https://crrev.com/e758a36b0255bba192d35cc14d2ffbedc019e029
Cr-Commit-Position: refs/heads/master@{#26747}

[Jakob Kummerow, 2015-02-19 12:18:35]

jkummerow@chromium.org changed reviewers:
+ jkummerow@chromium.org

[Jakob Kummerow, 2015-02-19 12:18:35]

LGTM with the comment updated as discussed offline.

[Toon Verwaest, 2015-02-19 12:24:18]

The CQ bit was checked by verwaest@chromium.org

[commit-bot: I haz the power, 2015-02-19 12:24:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/942493002/50001

[Toon Verwaest, 2015-02-19 12:28:09]

New patchsets have been uploaded after l-g-t-m from jkummerow@chromium.org

[Toon Verwaest, 2015-02-19 12:28:41]

The CQ bit was checked by verwaest@chromium.org

[commit-bot: I haz the power, 2015-02-19 12:28:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/942493002/70001

[commit-bot: I haz the power, 2015-02-19 13:01:57]

Committed patchset #5 (id:70001)

[commit-bot: I haz the power, 2015-02-19 13:02:09]

Patchset 5 (id:??) landed as
https://crrev.com/e758a36b0255bba192d35cc14d2ffbedc019e029
Cr-Commit-Position: refs/heads/master@{#26747}