Add tracing to saveLayer

I think this would be helpful for tracing as saveLayer is usually very costly.

Bug: skia:
Change-Id: Ia5bc194d6a7346c6fbec54da86065dc95d36f612
Reviewed-on: https://skia-review.googlesource.com/150115
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Auto-Submit: Yuqian Li <liyuqian@google.com>
1 file changed