refactored where texture descriptors are made for metal gpu backend

Bug: skia:
Change-Id: Ic90660b5ad5c1e795cd768bb7471e9ac80a59f47
Reviewed-on: https://skia-review.googlesource.com/138999
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
5 files changed