Move sync code to include/, switch from using platform define to a proxy header in core/

This fixes two problems:
  1)  #include SK_SOME_DEFINE doesn't work well for all our clients.
  2)  Things in include/ are #including things in src/, which we don't like.

TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/862983002
16 files changed