Cap SkVarAlloc's desired block at 64K.

This means we can store fLgMinSize in 4 bits (TBD).

Local perf comparison calls this harmless-to-slightly-helpful.  Nothing to get
excited about, but seems to certainly not harm perf.

BUG=skia:

Review URL: https://codereview.chromium.org/722293003
2 files changed