The GrContext's ID isn't really unique any more (since it can be shared among a family of contexts). Change its name to reflect the new reality.
Additionally, no client seems to be using it so make it private.
Change-Id: Ibb9004d699fe6ca7876b3be94142e612b5b9efbd
Reviewed-on: https://skia-review.googlesource.com/c/188308
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>