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=

Review URL: https://codereview.chromium.org/1296743003

Cr-Commit-Position: refs/heads/master@{#30839}