Revert of Revert of Simplify and compact transitions storage

Reland: Simplify and compact transitions storage

Original issue: https://codereview.chromium.org/980573002/

Simple transitions are now stored in a map's "transitions" field (as a WeakCell wrapping the target map); full TransitionArrays are used when that's not sufficient.
To encapsulate these storage format implementation details, functions for manipulating and querying transitions have been refactored to be static functions on the TransitionArray class, and take maps as inputs.

Committed: https://crrev.com/233ea0eef8510063ea8a27a2b8261b23bdd78a6b
Cr-Commit-Position: refs/heads/master@{#27044}

[Jakob Kummerow, 2015-03-06 12:33:49]

Created Revert of Revert of Simplify and compact transitions storage

[Jakob Kummerow, 2015-03-06 12:46:03]

PTAL.

Patch set 2 is the rebased version of the original commit. Patch set 3 is the
fix.

Only one line of the fix is important: at the end of TransitionArray::Insert(),
where an existing full TransitionArray is replaced with another one, the old one
must be zapped (this is not news, it was just an oversight in the original patch
that it forgot to do this). I've changed other calls to set_raw_transitions for
consistency.

[Toon Verwaest, 2015-03-06 13:23:32]

lgtm

[Toon Verwaest, 2015-03-06 13:23:57]

The CQ bit was checked by verwaest@chromium.org

[commit-bot: I haz the power, 2015-03-06 13:24:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/988703002/30004

[Jakob Kummerow, 2015-03-06 13:39:07]

The CQ bit was checked by jkummerow@chromium.org

[Jakob Kummerow, 2015-03-06 13:39:07]

The patchset sent to the CQ was uploaded after l-g-t-m from
verwaest@chromium.org
Link to the patchset: https://codereview.chromium.org/988703002/#ps270001
(title: "fix extra ';', again")

[commit-bot: I haz the power, 2015-03-06 13:39:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/988703002/270001

[Michael Achenbach, 2015-03-06 13:52:48]

FYI: CL description and subject are different. But never mind - I guess the CL
description is turned into the commit message...

[Jakob Kummerow, 2015-03-06 13:53:59]

On 2015/03/06 13:52:48, Michael Achenbach wrote:
> FYI: CL description and subject are different. But never mind - I guess the CL
> description is turned into the commit message...

Yes. I kept it that way (auto-generated title, manually edited description) so
that GMail succeeds in grouping the notifications. The commit message should be
fine.

[commit-bot: I haz the power, 2015-03-06 14:08:39]

Committed patchset #4 (id:270001)

[commit-bot: I haz the power, 2015-03-06 14:09:01]

Patchset 4 (id:??) landed as
https://crrev.com/233ea0eef8510063ea8a27a2b8261b23bdd78a6b
Cr-Commit-Position: refs/heads/master@{#27044}