Lazily register prototype users

Lazily register prototype users

when handing out validity cells to handles; because invalidating said cells is the only time we'll need the user registrations.
Along the way, fix a corner case in WeakFixedArray, which can now be empty after the recently introduced compaction support.

Committed: https://crrev.com/a4bb7643c076b014816431a9b85af3e2edf828e7
Cr-Commit-Position: refs/heads/master@{#28047}

[Jakob Kummerow, 2015-04-24 11:55:01]

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

[Jakob Kummerow, 2015-04-24 11:55:02]

PTAL.

[Yang, 2015-04-24 12:11:08]

lgtm.

[Jakob Kummerow, 2015-04-24 12:20:42]

The CQ bit was checked by jkummerow@chromium.org

[Jakob Kummerow, 2015-04-24 12:20:42]

The patchset sent to the CQ was uploaded after l-g-t-m from yangguo@chromium.org
Link to the patchset: https://codereview.chromium.org/1104813004/#ps20001
(title: "rebased (no conflicts)")

[commit-bot: I haz the power, 2015-04-24 12:20:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1104813004/20001

[commit-bot: I haz the power, 2015-04-24 12:52:00]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-24 12:52:17]

Patchset 2 (id:??) landed as
https://crrev.com/a4bb7643c076b014816431a9b85af3e2edf828e7
Cr-Commit-Position: refs/heads/master@{#28047}

[Jakob Kummerow, 2015-04-24 14:09:47]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/1108583002/ by jkummerow@chromium.org.

The reason for reverting is: Suspected of causing GC stress failures..