SkRecord performance is not sensitive to these values, so we can cut some
storage. This rearranges the space-remaining logic to use a count of bytes
left rather than a pointer to the end, cutting memory usage a little more.
An SkVarAlloc used to weigh 20 or 32 bytes which now becomes 16 or 24.
I think if I think about it a bit more I can trim off that Block* too,
getting us to 12 or 16 bytes.
Because we now just always grow by doubling, this CL switches from storing
fSmallest to its log base 2. This has the nice effect of never having to worry
about it overflowing, and means we can probably squeeze it down into a byte
if we want, even 6 bits.
BUG=skia:
Review URL: https://codereview.chromium.org/721313002
bc41538985