Reland "Lazily register prototype users..."

Reland "Lazily register prototype users..."

...when handing out validity cells to handlers; 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.

This reverts commit 968715c653b6337252a05a0224a7a93fab3b0866.

Original review: https://codereview.chromium.org/1104813004/

Committed: https://crrev.com/f6187fb3b52e518ccefd5de1bd1d4aca12bacba2
Cr-Commit-Position: refs/heads/master@{#28076}

[Jakob Kummerow, 2015-04-27 12:26:09]

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

[Jakob Kummerow, 2015-04-27 12:26:10]

PTAL. You've seen patch set 1 before.

[Yang, 2015-04-27 12:27:31]

lgtm

[Jakob Kummerow, 2015-04-27 12:27:44]

The CQ bit was checked by jkummerow@chromium.org

[commit-bot: I haz the power, 2015-04-27 12:27:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1110513002/40001

[commit-bot: I haz the power, 2015-04-27 13:00:02]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-04-27 13:00:18]

Patchset 3 (id:??) landed as
https://crrev.com/f6187fb3b52e518ccefd5de1bd1d4aca12bacba2
Cr-Commit-Position: refs/heads/master@{#28076}