Address stack overflow in SkPicturePriv::MakeFromBuffer

An SkPicture can contain an SkReadBuffer which can contain
an SkPicture which contains an SkPicture which can contain
an SkPicture which contains an SkPicture which can contain ...

https://review.skia.org/592736 added support for avoiding
deeply nested skps, but missed the case where a ReadBuffer got
tossed in the mix.

I had a test case for this in PS2, but removed it because it
was effectively just a huge opaque binary checked in and I
didn't want it to get stale eventually.

Bug: 496175229
Fixed: 496175229
Change-Id: I6183a78ed5a69499cb457d16d7e0fc430bff2b1f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1226696
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>