SkVarAlloc

Like SkChunkAlloc, but
  - does its allocation with better sympathy for malloc granularity;
  - the fast path inlines entirely;
  - smaller per-block overhead;
  - smaller per-SkVarAlloc overhead;
  - growth parameters are a little more tunable.

Its main downside is less flexibility; it supports fewer methods than SkChunkAlloc.

These current parameters bring the first allocation down from 4K to 1K,
without affecting recording time on my desktop.  skiaperf.com will tell the
whole story.

BUG=skia:

Review URL: https://codereview.chromium.org/674263002