Revert "Remove GrBackendRenderTargetDesc in favor of GrBackendRenderTarget."

This reverts commit e2d37c2a07f5473e5fc6fb65e9e23e14127580e9.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Remove GrBackendRenderTargetDesc in favor of GrBackendRenderTarget.
> 
> Also removes a reference to GrBackendTextureDesc in a comment and updates markdown docs.
> 
> Docs-Preview: https://skia.org/?cl=24861
> Bug: skia:
> Change-Id: Ic6490d5ef46953450e6dee69271397bb2b94d0d6
> Reviewed-on: https://skia-review.googlesource.com/24861
> Commit-Queue: Brian Salomon <bsalomon@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>

TBR=egdaniel@google.com,bsalomon@google.com,robertphillips@google.com

Change-Id: I4b85b529727f0bf5aec21d87e725a8195666e2e5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/25182
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
12 files changed