Introduce a base pointer field in FixedTypedArrayBase and teach GC about it

Introduce a base pointer field in FixedTypedArrayBase and teach GC about it

This is the first step towards unifying external and on-heap typed arrays.

The end-state will be that this base pointer either points to the on-heap values
or to the externally allocated array buffer.

BUG=v8:3996
R=hpayer@chromium.org
LOG=n

Committed: https://crrev.com/6cc3eb66a2b1b0885852c5ae3a58a4c3802d11fb
Cr-Commit-Position: refs/heads/master@{#29039}

[Hannes Payer (out of office), 2015-06-16 07:32:09]

LGTM, please explain in the description what the base pointer will be used for.

[jochen (gone - plz use gerrit), 2015-06-16 07:35:27]

done

[jochen (gone - plz use gerrit), 2015-06-16 07:35:29]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2015-06-16 07:35:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1176263004/1

[commit-bot: I haz the power, 2015-06-16 07:37:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-16 07:37:33]

Patchset 1 (id:??) landed as
https://crrev.com/6cc3eb66a2b1b0885852c5ae3a58a4c3802d11fb
Cr-Commit-Position: refs/heads/master@{#29039}