Adding V8 api to get memory statistics of spaces in V8::Heap.

This is first step towards adding V8 heap statistics to the memory
tracing infrastructure. For being able to get useful memory number into
the memory dump, v8 needs to provide an external api needs to obtain
more information about the heap. So, this Cl extends the api to give
information about the memory allocated and used in the spaces.

BUG=466141, 476013
LOG=Y

Review URL: https://codereview.chromium.org/1058253003

Cr-Commit-Position: refs/heads/master@{#27919}