Update BitField3 type in gen-postmortem-metadata.py

Update BitField3 type in gen-postmortem-metadata.py

Since https://codereview.chromium.org/272163002, BitField3 is a raw
uint32 field, and not a SMI anymore.

Update tools/gen-postmortem-metadata.py so that post-mortem tools can
work with versions of V8 that shipped after that change.

This change was merged in github.com/joyent/node right before node
v0.12.0 was released.

R=danno@chromium.org

TEST=mdb_v8, a post-mortem debugging tool running on SmartOS,  has been
using this change since Node.js v0.12.0 was released

BUG=

Committed: https://crrev.com/ff7d70bf414e2441c6ac14ba69840199840220c2
Cr-Commit-Position: refs/heads/master@{#30839}

[julien.gilli, 2015-09-05 06:40:37]

julien.gilli@joyent.com changed reviewers:
+ bmeurer@chromium.org, verwaest@chromium.org

[Benedikt Meurer, 2015-09-07 04:22:11]

lgtm

[Benedikt Meurer, 2015-09-10 17:39:28]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-09-10 17:39:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1296743003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1296743003/1

[commit-bot: I haz the power, 2015-09-10 17:39:43]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-10 17:39:44]

The author julien.gilli@joyent.com has not signed Google Contributor License
Agreement. Please visit https://cla.developers.google.com to sign and manage
CLA.

[julien.gilli, 2015-09-10 22:03:51]

On 2015/09/10 17:39:44, commit-bot: I haz the power wrote:
> The author mailto:julien.gilli@joyent.com has not signed Google Contributor
License
> Agreement. Please visit https://cla.developers.google.com to sign and manage
> CLA.

Signed, sorry for that.

[julien.gilli, 2015-09-18 22:08:40]

The CQ bit was checked by julien.gilli@joyent.com to run a CQ dry run

[commit-bot: I haz the power, 2015-09-18 22:08:52]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1296743003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1296743003/1

[commit-bot: I haz the power, 2015-09-18 22:35:09]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-18 22:35:10]

Dry run: This issue passed the CQ dry run.

[julien.gilli, 2015-09-18 22:39:42]

The CQ bit was checked by julien.gilli@joyent.com

[commit-bot: I haz the power, 2015-09-18 22:39:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1296743003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1296743003/1

[julien.gilli, 2015-09-18 22:40:55]

The CQ bit was unchecked by julien.gilli@joyent.com

[julien.gilli, 2015-09-18 22:40:56]

The CQ bit was checked by julien.gilli@joyent.com

[commit-bot: I haz the power, 2015-09-18 22:41:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-18 22:41:37]

Patchset 1 (id:??) landed as
https://crrev.com/ff7d70bf414e2441c6ac14ba69840199840220c2
Cr-Commit-Position: refs/heads/master@{#30839}