Have GrAtlasManager use legacy behavior

The old code would generate a 1024 x 2048 x 4 atlas given a
max size of 4MiB. And, the plot sizes would be 256 x 256. This CL
mimics that behavior.

Change-Id: Iae741d5409daa034c6c3086c8a09476fb343353d
Reviewed-on: https://skia-review.googlesource.com/c/162321
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed