Reland "In SkCanvas destructor, discard (rather than blit) unbalanced layers"

This is a reland of 879b2f2e6ebd7fb465c0679e22c48a0cae58c61e

Now includes a test that demonstrates the bug found by Chrome's fuzzers,
and a different (safer) implementation.

Original change's description:
> In SkCanvas destructor, discard (rather than blit) unbalanced layers
>
> Bug: skia:12267
> Change-Id: I6808f62b2385a3466b1a93db905041a6529f58cb
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/433360
> Commit-Queue: Brian Osman <brianosman@google.com>
> Reviewed-by: Florin Malita <fmalita@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Reviewed-by: Mike Reed <reed@google.com>

Bug: skia:12267
Change-Id: Ide7dc61b054761826faa5bca3eec6be2fc63c83a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/440977
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed