[skjson] Some short string optimizations

1) skip redundant \0 terminator => Value is always zero-initialized

2) skip storing a len record => strlen is cheap for short strings

Change-Id: I3c10c9b9cf6155b95124e2c0194c59e9531a7ca4
Reviewed-on: https://skia-review.googlesource.com/135049
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed