src: add total_available_size to v8 statistics

v8 introduced the new flag `total_available_size` in version 4.4
and upwards. This flag is now available on `v8.getHeapStatistics`
with the name `total_available_size`. It contains the total
available heap size of v8.

Introduced with commit: v8/v8-git-mirror@0a1352a7

PR-URL: https://github.com/nodejs/io.js/pull/2348


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>