[skjson] Remove the scope index stack

We already allocate a placeholder on the values stack -- we can use that
space to save the previous scope index instead of a dedicated stack.

Yields some minor perf improvements: ~3.5% arm32, ~0.5% x86_64.

Change-Id: I1be30aeb01b1c9661abfe06763a820673e3883f4
Reviewed-on: https://skia-review.googlesource.com/136178
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed