Move atlas manager creation to GrContext derived classes (take 2)

TBR=bsalomon@google.com
Change-Id: Ie10b7e770e24104d10c36ce7882126dd8551a8ba
Reviewed-on: https://skia-review.googlesource.com/110822
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed