We mark an object allocated as uninitialized. If we happen to have a GC before
fields of a map are written, msan will observe access to unitialized memory and
crash.

This also unifies the handling as we now deal with all spaces in the same way.
In future we could parallelize clearing.

BUG=chromium:638226
R=hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2251993002
Cr-Commit-Position: refs/heads/master@{#38681}