Correct capacity reporting for new space; show external size in Observatory.

Correct capacity reporting for new space; show external size in Observatory.

Before, the reported capacity included *both* semi-spaces, which seems wrong.

Also exclude external size from capacity, to be consistent with old space and
the comment on Heap::CapacityInWords.

R=johnmccutchan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=33625

[Cutch, 2014-03-12 22:39:58]

lgtm remember to follow the directions in HACKING.txt before doing the final
commit.

[koda, 2014-03-12 23:12:52]

Committed patchset #4 manually as r33625 (presubmit successful).