Allow TextureSamplers to have null GrTexture pointer

Bug: 715488

Change-Id: I69775cbb50d334d81872e236e59368fe65e698ff
Reviewed-on: https://skia-review.googlesource.com/14605
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
11 files changed