Tidy heap snapshotting implementation.

Adjust the division of labor between BaseHeap::takeSnapshot() and
the heap page implementations of takeSnapshot() --

 - have BaseHeap::takeSnapshot() handle creation of page dump objects
   (and their naming scheme.)
 - tally the heap free count+size in a separate object; only added to
   by "normal" heap pages.

R=
BUG=

Review URL: https://codereview.chromium.org/1700723002

Cr-Commit-Position: refs/heads/master@{#375455}
2 files changed