Revert "Remove the NullGL interface (and associated test context)"

This reverts commit de206c75c2c802f6f1e4ccb8a6b8a00765e9f085.

Reason for revert: Chrome is having issues with the switch to Mock in blink tests.

Original change's description:
> Remove the NullGL interface (and associated test context)
> 
> Bug: skia:
> Change-Id: Ie3c9ee39fc1e0a4406de085c60d8433ffb4419df
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/203708
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>

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

Change-Id: Iff0cbf29dcea26957efc800a8c33d0ad8285de0a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205343
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
15 files changed