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

This reverts commit 215da624d11340b254b5d17116bf20262f4ed69f.

Reason for revert: Blink issues ironed out.

Original change's description:
> 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>

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

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: skia:
Change-Id: I1f4fbbcb00f302c5d830cb1392badd6ec7a33c69
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205832
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
15 files changed