clean up SkPipe

One less SkCanvas subclass to deal with...

Change-Id: I21e81648026be5d732e8d9a28baed55015492a04
Reviewed-on: https://skia-review.googlesource.com/c/161584
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
12 files changed